Again, then...
Who do you nominate to decide who can be “in” the market, and who can’t?
True market forces only exist when you have a buyer and a seller. That's not what we have with medical care in the U.S. today. What we have is a patient who doesn't pay the bills and a doctor who gets paid by an insurance company or a government agency. Neither one of them has any incentive to conduct himself in a way that would reflect actual market forces. The patient has no incentive to be judicious about the care he receives, and the doctor has no incentive to keep the patient happy.