Why is it so hard to get into med school?

Now......I understand that getting into med. school requires outstanding GPA, MCAT score and outstanding curriculum. But what I really want to know is the statistical reasons why its so hard? Like is it because of the number of med. schools available? is it because of the government not wanting a influx of doctors? Is it because of......? What is it that makes it so hard!? If anybody who knows why plz respond and if you can leave a link of your source that would be amazing!! If not just leave a knowledgeable know reason why. NO BS, plz make your response off of legit reasonable known info. Links to your sources would be great!