Online voting could be vulnerable to such an attack but paper-based voting such as is used by mail in voting is very low tech and distributed. hacking and fraud are hard to scale in that case.

So, every mailbox in a post office gets one or two voter applications or ballots. A bunch of them get sorted out and thrown in the lobby trash can as people pick up their mail.

Some unscrupulous person grabs a bunch, forges signatures on them and mails them back in.