Voting by mail eliminates Republican poll watchers. Only county-level Democrat election officials in the 30 largest counties and the activists they hire would ever see or touch the ballots. This would go on unmonitored for several weeks, all day every day.
Unlike voting in person, where Republican poll watchers are present and for an entire single day.
This enables ballot replacement and ballot box stuffing, and when preliminary results show a loss, Dems can "harvest" however many ballots they need after the election.
Ballot harvesting is cover and a diversion to explain the hundreds of thousands of new votes. 99% of the new votes are just fake ballots created by unmonitored county election officials and their employees.