Well, there only so much in gaming that reasonably can be done server side.
Sure, the server could identify that a player shouldn't be visible and not transit that location to a client, addressing seeing through walls, in theory.
But once a player is hypothetically visible, aimbot can happen. If you are crawling in a ghillie suit in the grass, but the other player has a client that skips rendering grass and replaces the ghillie suit model with a suit made of traffic cones...
Now intrusive anti cheat isn't worth it, but it is an unavoidable reality that it is up to the client to preserve the integrity.
Closest you get would be streamed gameplay, where the rendering even is server side. Also not worth it. But even then I could see cheating machine vision and faked controls to get an edge unfairly.