That and trusting client information with extraordinarily few server-side validation such as, for instance, impossible shots, impossible movements, impossible s

That and trusting client information with extraordinarily few server-side validation such as, for instance, impossible shots, impossible movements, impossible speed, etc. It does no client information culling (beyond a large circle of information around the player's position) enabling wallhacks, aimhacks, etc. These are hard problems to solve on the server side, and require resources, so it was easier not to worry about it at all, and at most do post-game analysis occasionally to try to retroactively detect the impossible (with a very low detection rate, so much so that they rely upon user reports). This is one of the selling points of something like Stadia (which has to be balanced with the downside of latency) -- by default a cloud renderer would give minimal information to the client.