The goal of anti-cheat isn't to stop the world's most advanced cheaters. Those are already unstoppable because they now use Direct Memory Access over the PCI-E bus, so the cheats don't even run on the same computer anymore. However since those cheaters are few and far in-between they can be handled through player reports.
The goal is to stop the mediocre cheater who simply downloaded a known cheat from a cheating forum. If you don't stop those you'll get such a large wave of cheaters that you can't keep up with banning them quickly enough.