Because voting is anonymousdone by secret ballot. If a fraudulent ballot is separated from its envelope, there’s no way to invalidate it.
When voting by mail, your ballot is in two envelopes. The outer envelope contains your name, signature, postmark, and whatever other mechanisms the state uses to validate your identity and your eligibility to vote. The inner envelope contains no identifying markings whatsoever.
Once the eligibility has been validated, the two envelopes are separated from each other so it’s not possible for a pollster to determine who voted for whom.
If it turns out that fraudulent votes were cast, there’s no way to adjust the vote counts accordingly, because there’s no way to tell who the fraudulent votes were cast for.
Using PA as an example, PA state law says that ballots must be received no later than 8PM on election day, but the PA Supreme Court ruled that mail-in ballots can be counted up to 9 days after the election to account for mail delays, even those without a postmark date.
The Trump campaign asserts that the judge’s ruling is illegal and wants to appeal the decision. If the votes have already been counted, then there’s no way to undo those counts even if they prevail in court. Therefore, they want to stop mail-in ballots that were received after election day from being counted, because that’s the only way they can prevent them from being included in vote totals if they win their case.