Without root/admin access, on windows programs can't write in several important folders. By root user they meant program files, system 32 and all those "system files", which, surprise, are root files.

A hacked kernel level program can modify system files and set up a keylogger that doesn't even register on the program monitor, and it can send your information and you wouldn't even notice it without monitoring your outbound packets, so you won't.

Any other program would ask you admin/root access and if that's weird behaviour you can deny it and investigate, kernel level programs have it by default so if they have an exploitable vulnerability, you are fucked by default. It's a huge difference and the fact that you are not acknowledging it makes me feel like you really don't understand how code works.

Also, don't put riot and valve in the same bag. PLEASE.

No, on Linux, like the Steam Deck that OP mentioned. Windows was never mentioned here until you brought it up, and even there you're wrong, kernel level anti-cheat doesn't have the same level of access than any random game, even on Windows. Even Windows, with it's janky security measures, has some level of containment around users, even on Windows regular users can't edit system files or other users files, even on Windows a virus without root has a lot less access than a virus you give root access, and by having an interface that allows games to gain root access you've given viruses a new path to privesc. I recommend you read some more on cyber security and programming before saying something like "userspace == kernel level", because that's the same as someone attempting to discuss astrophysics with people who have masters on it while claiming the earth is flat. There's a whole field of study into how security can be compromised to go from userspace to kernel level, handwaving it away because you think your user's documents are the important part of a system is reductive at best and malicious at worst.