Because human moderation after an AI serverside filter is much more consistent and less desperate than solving the halting problem clientside 2 rings below your app's privilege level.
If you're watching the game as a moderator you could never tell that's what's happening. Yet -20% recoil, especially for a competitive esports player, would be such a massive advantage as to make that player the best in the world, but a way that looks quite legitimate.
Especially since the cheats need to be available somewhere, so the developers should certainly be able to get their hands on them to test...
I'd also suggest that human moderation is definitely less consistent, and definitely less able to scale.
Nobody is trying to solve the halting problem in the kernel either. If a cheat runs as a kernel level driver, and anticheat doesnt you've lost the battle already.
It makes sense the cheat I wrote would run as admin. It's my PC after all. I also don't have access to the server in any meaningful way and modify other people's data, either. It's a good trade-off. Devs can also write arbitrarily good statistical analyses for the game. It's their responsibility.
The question is simple - is this person performing abnormally? Well, games gather tons of telemetry nowadays, and the harder-to-detect cheats like wallhacks or fluorescent player textures are also the least useful. I am a hard silver I in CSGO. With or without aids, when I see someone run around the corner, I'll duck and magdump. Any gold nova will overpower me either way.