Mostly because actually spotting cheaters takes a lot of game knowledge and experience. In FPS games anyone can spot a spinbot, but how you can be sure that someone uses wallhack? or aimbot that just slightly corrects the aim?
Without proper game knowledge you wouldn't be able to distinguish a wallhack from someone listening to footsteps, having info from teammates, and having a good map awareness.
It is very easy to cry 'cheater', especially without any actual proof. Without knowledge that, for example in CS: spray patterns exist, even someone correcting them would look like a cheater to a newbie.
I spent about 300 hours lately playing new COD, and there was only one case where i suspected someone of cheating. One.
And i do play a lot of FPS, and played them for good 25+ years.