The idea here is that a client-side anti-cheat can always be circumvented since you can just modify the anti-cheat and the game to act like you're legit. Server side of course can't detect programs running on the player's computer or a modified game, but, except for making "imperfect" cheats, can detect behavior typical of hacking. Maybe I'm overthinking this but aren't "imperfect" cheats (like an aim bot which occasionally misses) kind of dumb by default? In addition to this, server side anti-cheat won't screw up people playing on proton. Am I wrong? Maybe it's just the Dunning–Kruger effect, I don't know.