There’s a lot of work that needs to be done for mail-in votes. I only have first-hand experience with the German voting system, but from what I read the US system is similar: The actual ballot is in a sealed envelope. This envelope together with a signed paper (“Wahlschein” in Germany) is in a second envelope that goes in the mail. The process of counting that vote starts with opening the outer envelope and verifying that the ballot was issued to the person signing. Then verify that the inner envelope is properly sealed to ensure that the vote is actually secret. Collect the inner envelope with the actual filled-in ballot. No counting has happened yet, but this verification costs considerable time. All of this could be done even before Election Day with no effect on voter suppression - no one at that point can actually know the vote.

In theory, even counting could be done as long as no result is leaked, though there’d be a risk.

Explicitly not allowing the preprocessing - and as far as I followed the news, that’s what happened in PA - seems like a pretty clear cut case of “we want to draw the count out.” to me.