What would be the point of a competitive game if you could just cheat? Racing games like Trackmania live off verifying people don't cheat because it's a game where people compete for leaderboards. Should they not be allowed to prevent you from cheating yourself to the top of the leaderboard.
But it's not impossible to prevent cheating. CS:GO has a very low occurence rate of cheating as most cheaters are caught extremely early by anti-cheat or later by other anti-cheat efforts with more complex functionality.
They do in fact verify server side that everything is where it should be. But it doesn't stop people because ultimately the inputs of a high level player in Trackmania are nearly indistinguishable from inputs of a bot playing the track.
CS:GO faces the same issue where inputs by high level players are indistinguishable from cheating.
Speedrunning tends to have similar issues, where people perform tricks that are so close to TAS inputs that it can take years to spot an issue. Or in case of Dream, it takes an entire statistics paper to explain why they cheated. In those cases it's even worse because you cannot do server side validation on video data.
Being against anti-cheat is effectively being against competitive gaming or speedrunning.
