This is probably true with the current setup, but traditionally doctors had a particular social position and professional mythology around them. I'm not sure if it was ever really true that they didn't respond to economic incentives, but the perception (both external and among doctors themselves) was that they had a sort of above-the-fray perspective where they were supposed to use their learned judgment for the betterment of those under their care, rather than engage primarily in business. Sort of the same thing expected of priests, professors, sheriffs, etc.