To allow time for them to arrive in the mail?
You are correct.They need time to travel through the mail. Here in New York, absentee ballots must be postmarked by midnight the day before election day, or delivered in person to the County Board of Elections on election day. For military ballots, the delivery deadline is somewhat murky (ie longer), But the general public’s absentee ballots must be given a week or so to travel through the mail. Absentee ballots are counted in the voting totals, however emergency paper ballots are not unless the number of votes needed to determine the margin of victory in the race have been cast on those paper ballots.