Trump claims the election was stolen, via the manufacture of votes for Biden.
Anyone who has worked as an elector knows that vote manufacture is easy to prove - every vote comes from a precinct, and it is trivial to trace any vote back to the precinct, and compare against the poll book (if the voter voted in person).
As for absentee ballots, if they are pre-printed fakes, then the risk would be that some people who actually mailed in their ballots would also have pre-printed ballots in their name. In other words, an easily discoverable mess.