Ballot reconciliation ensures that every ballot printed is tracked, as well as ballots counted and ballots cast match up.

And as it is spread out over counties and districts, any injection anywhere would fail those checks.

I'm not an expert, and there is likely at lot more depth to this, but I did try to read up on it after the 2020 election and was sufficiently convinced at the time at least, but I'm happy for anyone to correct me.