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.
They try to make own matchmaking for possible statistical outliers so cheaters end up playing against each other. Of course, real good players can still get there and there are (at least used to) real humans on reviewing on those games to see if someone is actually a cheater. It is not a simple task, since you can cheat to be just slightly better than others and that is enough to be good.
So I think it is fair to say the there haven't been billions of dollars of research spent on this problem.
[1] https://www.dexerto.com/gaming/anti-cheat-study-reveals-chea...
There are also the costs of the opportunities that cheating prevents from happening. Development would be much faster and more types of games could be made.
[1] https://www.scmp.com/tech/apps-social/article/3099893/chinas...
Edit: I missed that you are the same commenter that I replied. sorry about that