Why has the number of MD's produced by the nations medical schools been flat for 20 years?

Why has the number of Medical Doctors produced each year by the nations medical schools been relatively flat for 30(!) or so years?  This question is raised by Professor Mark Perry at Carpe Diem.

From USA TODAY: ""The marketplace doesn't determine how many doctors the nation has, as it does for engineers, pilots and other professions. The number of doctors is a political decision, heavily influenced by doctors themselves.  Congress controls the supply of physicians by how much federal funding it provides for medical residencies — the graduate training required of all doctors.
The United States stopped opening medical schools in the 1980s because of the predicted surplus of doctors. The Association of American Medical Colleges dropped this long-standing view in 2002 with the statement: "It now appears that those predictions may be in error." Last month, it recommended increasing the number of U.S. medical students by 15%. ""

He who controls the supply can dictate the price (Isn't this what the oil cartel OPEC does with oil prices?). What organization is a generous supporter of Federal elected officials and has one of the most powerful lobbying groups in the US?  The American Medical Association (AMA)...The Central Planners at the AMA who have input into the opening of new medical schools failed to predict the market need for Doctors...hmmm...I suppose only the AMA can perform surgery on an invisible hand and declare the patient healed.