As President Trump and others have said repeatedly, changing to what amounts to a vote by mail election without a lot of preparation by the state governments leads to a situation where hundreds of thousands of ballots are just floating around. For every ballot addressed to someone who used to be a voter, and sent to where they used to live that gets returned some fraction don't. And those ballots may well have ended up being submitted by someone else.
That is a simpler explanation for the dead people that have voted than those people resurrecting from the dead to vote.
Why would this drive down the number of rejected ballots?