It's much harder to sell a cheat that requires the user to significantly alter their computers boot process. Anti-cheat just exists to inconvenience cheaters enough so that the cheats lose their value.

It makes the cheats more valuable on the black market. I'm fairly sure the only people cheating in the major competitive games with anticheat are whales and extremely unethical pro players.

If that's the case then why not only have kernel level anti-cheat enforced for the leagues and the tournaments?

Because then a lot more people would cheat outside of leagues and tournaments.

That's not really true if the exploit requires soldering on to RAM pins and executing on a second, independent machine.