> Because doctors' associations and regulatory bodies (like AAMC in the US) lobby to keep it that way to keep the value of their profession up.
This isn't true of the AAMC position in the US today, and when it was true in the 90s, there were many articles about an upcoming oversupply of physicians.
First, US medical school graduating classes are smaller in number than the number of available residency positions. So every year, the US is importing physicians trained in other countries.
Next, residency positions (required to practice in the US) are funded by the US government. You could readily contact your US representatives about the problem you perceive - if this a legitimate concern for you.
Additionally, US residency positions don't need to be funded by any government body, at all! Hospitals need 'simply' show that there is enough patient volume to support educating additional residents. This is another avenue where you can intervene, if this is indeed something you care about.
Lastly, 'advanced practice providers' are filling in large amounts of the deficits in physicians in primary care providers. So focusing on the number of physicians is to ignore the huge growth of NPs and PAs - some of whom can function without a physician in some parts of their practice.
I see many people blame 'the AAMC' for healthcare problems, but worry that not many appreciate the lack of a role the AAMC plays in the number of providers in America.