It's because the NIH limits the amounts of medical residencies in the country, and to become a doctor with a license to practice, you must have completed residency somewhere. The amount of doctors in the US are kept at low numbers because of this.
The option almost everyone picks is “no one”. Keep supply of your services low and increasing demand means more money for you.
The AMA is actually doing a great job at this recently compared to the ABA, where there have been too many lawyers graduating recently for insane salaries except at the top.
[0] https://www.npr.org/2018/03/09/592333771/severe-shortage-of-...