The biggest reason is to implement the rather complicated algorithm known as E Pluribus Hugo, or Single Divisible Vote with Least Popular Elimination. Also, this is a volunteer job and historically there hasn't been any ability to summon the necessary personpower even before EPH.

That is literally how Australian elections work, and we vote by pencil on paper and count by hand. Takes a few hours to get results across the nation, a week or so to double check. And Australia is famous for the integrity and accuracy of our elections.