According to a handy map by CNN, Pennsylvania, Wisconsin, Mississippi and Alabama do not allow pre-counting of mail ballots. Why is this the case? Why not count mail ballots as soon as possible to report the results quickly on election day?
In past elections, mail-in ballots were an afterthought for most states. There weren't that many of them, so you could normally tell what the election results were before even counting them. Even if you did need to count them in order to find out the result of an election, counting them was pretty quick because there weren't very many. Therefore pre-canvassing and pre-counting were not issues anyone cared about very much, and states just kept on doing whatever random procedure they'd been doing for decades.
In 2020, there have been several big changes compared to previous years: (1) because of covid, the number of mail-in ballots was much greater; (2) there has been a politicization of the public health response to the epidemic, including precautions like masks and casting votes by mail; (3) because of this politicization, mail-in ballots in a swing state like Pennsylvania were heavily Democratic, while in-person votes were heavily Republican.
All of this happened against the backdrop of ongoing attempts by the Republican party at voter suppression, along with false claims by Trump and some others on the right that voter fraud was a widespread problem.
Once the situation became clear, people knew that there would be an intense "blue shift" in states like Pennsylvania, meaning that the in-person votes would be counted first and would favor Trump, while the mail-in votes, counted later, would gradually shift the result the other way. This had already been observed on a smaller scale in 2018, when, e.g., Gil Cisneros was behind in his congressional race on election night but ended up winning.
As it became more and more clear that Biden was heavily favored to win, Trump began preparing an alternative narrative that would allow him to claim that he had won, but the election had been "stolen." A blueshift would fit nicely with this narrative: Trump could declare victory on election night, and then when the blue shift happened and he lost, he could claim that the mail-in votes, counted later, were fraudulent. The narrative would be much less compelling if Biden built up a huge lead in mail-in ballots that were counted early, and then in-person voting failed to bring Trump back from behind.
The most prominent issue was in the crucial swing state of Pennsylvania, where the Democratic secretary of state begged to be allowed to pre-canvass mail-in ballots, meaning that election workers would go through the time-consuming process of opening the envelopes and getting the ballots ready to count. After that, counting the ballots would be easy. Republicans in the state legislature did pass legislation that would have allowed pre-canvassing, but passed it as part of a bill containing many other measures that made it a poison pill for the Democratic governor.
So in summary, there is not a rationally defensible reason for forbidding pre-canvassing, and it's also very questionable whether there is any rational reason for not pre-counting (quote from news coverage):
Former Pennsylvania Gov. Tom Ridge, a Republican who recently announced his support for Joe Biden, said it doesn't make any sense that allowing for pre-canvassing in his state has been such a struggle.
"We want an accurate count. We want a legitimate vote," said Ridge, who co-chairs the bipartisan VoteSafe group. "And we want it done as quickly and as efficiently as possible. If that's the goal, give them time to do it and start pre-canvassing several days before election day."
The way this has played out has been entirely a function of the non-rational appeals that Trumpism has made to voters who don't get their information from reliable sources, and who are therefore willing to believe in conspiracy theories such as birtherism and widespread voting fraud.
An answer by Colin says:
This is usually to avoid the leaking of any partial vote tallies before the election, which might have the effect of unfairly encouraging or discouraging turnout of voters favoring one candidate or another.
This would fail to explain why pre-canvassing was not allowed in Pennsylvania. Pre-canvassing would have meant not counting any mail-in votes before election day, only getting them ready for processing.
One reason is the scenario of a person casting a mail-in and then coming in and casting a vote in person. You don't want that double-counted. It's easier to set aside a ballot if the person is in the system as having cast a vote earlier in the day, at the time of opening the ballot than going back and trying to retrieve that ballot from somewhere, wherever it may be, when the person shows up at their polling place. Absentees are usually/often handled in centralized locations for the district, and not necessarily the voters' specific polling stations.
Also, if there is an issue of voter impersonation via mail-in, or a broader election fraud via bogus mail-ins, with a person showing up in person being more verifiably authentic, their in-person vote would be counted and their fake mail-in would be chucked or set aside as provisional when it came time to tally those ballots, and that person is listed as already voting, in person. The legitimate, in-person voter would not (or would have less of a chance) have their authentic vote being thrown out or canceled in some way.
I'm not saying that there aren't other ways to deal with it, like retroactively flagging certain ballots and making the provisional after the fact, but that's not as simple or straightforward. Making that easier to administrate and verify via the "just wait until voting is done to deal with those" probably made a lot more sense in election cycles where mail-ins were more an outlier than the norm, which is when these rules were put into place.
To supplement the answer with details on Pennsylvania, the Republicans who controlled the legislature there tried to bargain with the Democratic governor over pre-canvassing (pre-counting) in exchange for what they saw as necessary changes in the law, like relaxing residency restrictions on poll watchers and "banning mail drop boxes". These negotiations broke down, and so Republicans dismissed pre-canvassing as unnecessary:
Under the law, mail ballots cannot be processed until Election Day. But for a moment in June, it seemed as if the stars aligned. The GOP[-controlled] House advanced an election reform bill that would allow pre-canvassing to begin 21 days before Election Day.
By the time the state House approved the bill, however, the window was shortened from 21 days to three. And then came what Democrats considered poison pills: Rather than passing clean legislation, Republicans wanted unrelated concessions, including relaxing residency restrictions for partisan poll watchers and banning mail drop boxes — and the latter was a nonstarter for Democrats.
[Democratic governor Tom] Wolf said he would veto the bill.
[Kevin] Boyle [(D., Philadelphia)], also minority chair of the House State Government Committee, said he tried several times to introduce amendments to allow for pre-canvassing. He also put up a clean pre-canvassing bill, but it was not considered.
On Wednesday, Senate Majority Leader Jake Corman (R., Centre) told reporters his caucus did not continue working on legislation because lawmakers didn’t trust the state’s top election official, Kathy Boockvar, to implement the measure properly. Corman and the Senate’s other top Republican, Joe Scarnati, are calling on Boockvar to resign over guidance she provided to counties ahead of Election Day.
Corman also said Wolf “needs to take some responsibility” for vowing to veto the House measure.
“That’s not the issue of the day, that’s the issue that they want you to talk about,” he said of pre-canvassing. “Most counties didn’t need it. I think we’re fine.”
tl;dr: To answer why not all states pre-count votes, this answer lays out what voting processes that do enable pre-counting have to give up and what they gain, compared with those that don't have pre-counting. The conclusion is that both approaches have their own advantages, neither is clearly better, and thus both approaches make sense.
In theory*, there is a very simple process that doesn't require pre-counting:
- Until election day collect mail in ballots and lock them into sealed boxes.
- Early in person votes work the same as mail-in ballots and are also locked into sealed boxes.
- When the polls close, open the boxes, open the envelopes, verify that the signature matches, voter is still alive, eligible to vote, etc. If valid, throw the inner envelope (that contains the actual vote) on the pile to be counted.
- Count the votes.
That process has a number of good things about it:
- It's simple to execute, and difficult to mess up.
- It maintains the anonymity of the ballot (which is a hard requirement in most Democracies)
- It allows to only count votes by voters who are eligible to vote on election day
- Doesn't require election observers to observe counting before election day, just to observe the boxes are sealed.
- Early results cannot be leaked before election day.
- As an optional benefit, the process allows for people's in-person vote to override whatever vote they mailed in earlier, which allows them to change their mind, and disables a form of accidental double voting without criminalizing it.
If you count votes early, we can only have either 2) or 3), and we lose 4), 5), and 6). 1) depends on the specific implementations we're comparing. The single advantage of early counting - earlier results - can be mimicked by hiring more people to count, or effective use of automation. But since there is disagreement about the value of having early results, increased spending to produce faster results is not a universally adopted practice.
*In practice there are quite a few different variations of this and other processes, each with their own benefits and drawbacks. For example in order to allow voters who sent in invalid ballots before election day to correct them in time, some states peek at the info in the envelopes - which requires the presence of election observers. So a specific question about a specific law in a specific state may end up getting a different answer.