Because what do you think matters more in an online game:

A. Smooth and consistent client experience, where bullets hit what you aim at (client-side prediction) where aimbots and wallhacks work.

B. Jittery/laggy client experience, where aimbots still work, but wallhacks are disabled?

You can only choose one option.

Generally, everyone agrees "A" is the best option and cheaters will be dealt with at game time. It's annoying, but that's the cost of online video games.

Also even if it worked perfectly, there'd still be other practical ways to cheat. In a way it's better to detect cheats than prevent them, because you waste a lot more of their time sticking them into a pool of cheaters and can also undo it. Though afaik Counterstrike tells you when you're low-trust.