When my health is on the line, I do not want to do anything like shopping around. I want the hospital I'm currently at to do the thing for a reasonable rate. Preferably the same rate as every other hospital in the country charges for the same scan.
How do you obtain food then? Do you not go to grocery stores or restaurants?
But talking about just medical services, sure there are many situations where a patient has vanishingly little opportunity to shop around, and applying the market ethic is really just a blessing to price gouge. But there are also many situations that aren't so time critical (like anything that insurance companies currently insist on "authorizing"), where it would be quite easy to shop around if providers had to charge everyone the same price and publish it.