How much "investigation" is going to be needed to satisfy you?
How is such an investigation supposed to occur, after the fact, if it was not identified before the fact?
That is the problem inherent with mail-in ballots. Unless it is actually caught occurring, as was the case reported in Texas in 2020
...:
She allegedly submitted voter registration applications for 67 residents without their signature or effective consent, while purporting to act as their agent.
www.cbsnews.com
...It stays under the radar until too late if at all. Like this example of a recent audit of Wisconsin nursing homes:
It's kind of interesting that there was an entire era in American entertainment in which bad people had goatees. If you're a villain, you probably shouldn't get a goatee. Every time you enter a room, everybody says, "That's the bad guy. Protect your wallet." Did you ever see the South Park...
debatepolitics.com
So, yes it does happen, and IMO quite a lot more often than you and others are willing to credit.