Dude....they don't save 'mail in ballots' for last...
Depends on the state. In some, mail-in/absentee votes are counted before Election Day. But in most, they aren't. In many, counting can't begin until after the polls close. And that's why there're big dumps in the middle of the night.
I'll add that typically, military votes, which are almost all absentee, are often not counted till after Election Day. Think we should just throw 'em out?