The entire Steam account is tainted: that's the issue.

Some random commercial third party can make an accusation and damage the value of thousands of games on a lark.

Meanwhile, any determined cheater just bought another copy of the game on an account dedicated solely to that task. This person suffers no extended consequence.

With Family accounts it's even worse: the tainting is attached to the account owning the licence of the game, not to the account playing the game. So if you share a game with a kid and (s)he's caught cheating, your account will be tainted, not just the kid's.