I have voted absentee a few times here in my Minneapolis suburb. I had to fill out and sign a thing and send it in if I recall correctly. They sent me a ballot, I had to sign it and mail it back.
The mail in scam would be just a pile of ballots sent to every mailbox and available everywhere. Massive fraud would be easy.
Even if they actually address the ballot to every registered voter, how many of those are going to be dead, no longer in the state, or otherwise ineligible to vote?
If you live in the average apartment building, when someone moves out without a forwarding address, their mail just piles up in the lobby under the mailboxes - ripe for anyone to go around and harvest, and use in whatever way they want.
In my state, to vote absentee, you have to request an absentee ballot, explaining your reason/need, and they send it to you. It’s not just a matter of mailing out ballots to everyone, as mail-in voting would be.
Someone else here noted that pre-paid mail, as this would probably be, isn’t tracked by the post office. How do you know if your ballot even made it to its destination, or when? Who is going to keep a record, and how, if you want to track whether your vote was registered?
There are lots of reasons why mail-in voting is dangerous - fraud, long delays in counting and knowing results; and lots of legal disputes will arise.