Well I assume there are ways to prevent that (or make it extremely difficult at least)? E.g. look at denuvo, nobody has been able to "open a hex editor" and disable denuvo.
Not the recent denuvo versions, and not in the past ~1 year.
That's not even the point though, I am not saying it is literally impossible to circumvent this, but as long as it is hard enough that it is not financially reasonable for the cheat makers, that's good enough.
Denuvo has been disabled many times; however, the amount of work required to modify all the generated injection points is tedious—it's a LOT of work. It seems that fewer and fewer people are willing to spend weeks or months of their lives cracking a single game.
Anti-cheat systems, on the other hand, are entirely different. If you only need to modify one variable in the game, it's much easier because, in most cases, that variable is frequently used. This means you can't add too much overhead to its use, and after all, it's just one variable.
Denuvo isn't just a flag on a process. It's no more relevant to your suggestion than encryption would be to a suggestion that audio files have an option to prevent them being copied.
In fact, three people have been able to do so, that's why denuvo games do get cracked.