I feel like this is the same as saying "seatbelts don't prevent car accident deaths at all", just because people still die in car accidents while wearing seat belts.
Just because something isn't 100% effective doesn't mean it doesn't provide value. There is a LOT less cheating in games with good anti-cheat, and it is much more pleasant to play those games because of it. There is a benefit to making it harder to cheat, even if it doesn't make it impossible.