When everyone is healthy, a doctor can’t make money. Therefore, doctors have an incentive to do Bad Medicine in order to keep the population at least partly sick.
However, it’s a Tragedy of the Commons. It only works if all doctors cooperate to keep the population sick. An individual doctor has an incentive to defect by doing Good Medicine. They can gain more reputation and thus money this way, and also some self-respect, and leave all the other suckers to do the hard work of keeping the population sick. By Game Theory, all doctors gradually defect, and the entire population ends up healthy.
THANKS A BUNCH, Game Theory.