Because the same way your day can get fucked up for cheating (deserved) is the exact same way your day can get fucked up for cybercrime reasons.
Do you really trust companies with modern hiring standards with that level of access to your computer?
The Crowdstrike incident that happened within living memory was made possible by the fact that Crowdstrike has kernel level access, just like Riot Vanguard anti-cheat. This essentially shut down a good part of the corporate sector worldwide, along with airports, rail hubs and so on, for a day or two. It would have taken a lot longer to solve if it wasn't a mistake from an otherwise good company with plenty of actual competent programmers to throw at the problem.
What if next time it's not even a jeetcode error? I can't think of a much better way for a bad actor (state-sponsored or not) to get access to a bunch of high-powered PCs for malicious purposes at once than managing to hijack anti-cheat software for a game.
Here's another example: in 2017, the biggest freight company in the world,
Maersk, got shut down so hard that they only managed to get back their ransomware'd database because an agency of theirs in Africa hadn't connected to the Internet for a couple of weeks.
How did this happen? An automatic update to a word processing software a branch used. Again, global shipping ground to a halt for two weeks because of an automatic update.