So you are saying a foot fault has never happened inside the circle? and players call it every single time? no confusion across masses of players or courses etc? ....... come on'
Foot faults happen, and oftentimes go uncalled. But the 10m circle is easily enforced. It's not exact down to the centimeter, but at least you can always get a group consensus.
Determining if a lie is "in or out" is easier than policing falling/jump putts.