> You already know where an enemy is if you hear them behind the wall,

You know they are somewhere behind the wall, you don't know which exact angle they are behind the wall, because headphones and our ears don't work with that degree of accuracy.

The cheater can just swing the corner with his cursor already pre-positioned exactly on his target. Between peeker's advantage (inherent to any online game with latency) and human reaction time, there's not a lot you can do to fight that.