The baseline seems to be 3000 ballots per hour. Once a ballot is fed into a machine and a result is tabulated, it has to be audited, then certified, then passed off to a reporting authority. Also, volunteers need to eat, sleep, use the bathroom, etc.
Blame the legislatures that didn't think pre-processing of mail-in ballots was necessary or even valuable.
EDIT: FWIW, the race is essentially over at this point and Biden has won. The voting percentages in the remaining states are not breaking in a way that suggests an upset is even possible, let alone likely.