The problem is that most cheaters don't just go full aimbot and track people through walls. That is a surefire way to make sure your account gets reported, reviewed, and banned regardless of what anti-cheat is in place.

Serial cheaters cheat just enough to give themselves an edge without making it obvious to the people watching them. By just looking at their stats, it can become very difficult (though not impossible) to differentiate a cheater from a pro player. This difficulty increases the odds of getting a false positive, necessitating a higher detection threshhold to avoid banning innocent players.