> But why doesn't this happen to restaurants, or super markets or car mechanics?

There's plenty of examples against your case already in the comments against other points but the reason medical care is so damn expensive is the same reason your internet bill is so expensive (for poor quality): lack of choice.

You don't really have an option to haggle or shop around for the best plan when you broke a leg and are being rushed to the nearest hospital.

You made the choice on plan half year ago when you were healthy and you're in no state to have a calm discussion with the ambulance about what has the best price per value with your tibia jutting out of where your calf should be.