Looking at the New York Times election map, one can see that Arizona, Georgia and North Carolina have not been called yet for Trump or Biden despite having counted nearly every vote by now. Alaska hasn't been called either, but they've only counted 61% of the votes so it makes sense.
Why is this the case? Why not call NC for Trump and Arizona/Georgia for Biden? What are these news outlets waiting for?
The answers for these questions are drawn largely from the 538 Uncalled Races live blog:
Georgia has not been called because a recount is pending. Even though recounts almost never change the results, most news outlets have a policy of not calling a race that is under a mandatory recount, or a requested recount if the margin is very small.
The AP does not declare a winner of an election that will be — or is likely to become — subject to a mandatory recount. In instances where a recount isn't required by law but a candidate requests one, AP will not call a race if the margin between the top two candidates is 0.5 percentage points or less.
In Georgia, the margin between the candidates is only 0.25%, below the AP's threshold to call while a recount is underway. In Wisconsin, by contrast, Biden leads by 0.62%, so their policy allows them to make the call, despite the Trump Campaign's pending recount request.
North Carolina allows mail-in ballots to be received up to November 12th, so long as they were postmarked by election day. So we don't know the final tally of how many votes are still to be counted until then. It's extremely unlikely that there will be enough votes out there to change the race, but it's technically possible and so the uncertainty is keeping the race from being called.
Arizona is still counting the last few mail-in and provisional ballots. Since these ballots have been trending Republican, it is still possible for Trump to win. This seems unlikely at this point, as Trump has not been winning these votes by the >62% margin he'd need to win, but the race will be close.
It's a similar issue with Alaska: There are between 145k and 156k mail-in ballots which have not been counted yet, and since we haven't started counting them, we have no idea what kind of margin Biden might have. A Democratic win seems unlikely, as Biden would need to win 67% and Al Gross (the Senate Candidate) would need to win 68% of the uncounted ballots, but those are lower margins than what Biden got from mail-in ballots in Pennsylvania, so it is technically possible for Biden to win – hence no call. For what it's worth, Al Gross claims that his data suggest a Democratic win is possible.
In Arizona and NC there's of a combination of 2 different issues:
In Arizona Biden is 0.44% ahead with ~2% votes outstanding.
In North Carolina Trump is 1.3% ahead with ~2% votes outstanding, and the last batches of ballots having been strongly in favor of Biden.
In Georgia Biden is only 0.25% ahead, which triggered a recount that hypothetically could change the outcome.
Other networks have called the election because they use different definitions of sufficiently plausible, significantly, close enough, and/or strong.
I didn't bother looking up the specific method used by NYT, but I read somewhere (might be for AP) that they might delay calling if the margin between the two major parties is below 0.5% (or some state-individual recount threshold) even if the remaining votes would have to be extremely different from the trend (even compared to the expected large difference between direct and mail-in voting, or between fast-counting rural and slow-counting urban areas) in order to flip the result. This would explain why they didn't call Arizona and Georgia yet. Calling North Carolina for Trump would be more realistic by that rationale - but note that they are still waiting there for slowly delivered mail-in ballots unto Nov 12
