Even between doctors different specialities fight each other to maintain a monopoly on certain procedures, like gastroenterologists and general/gastric surgeon with endoscopy, or any time general practitioners take on a more specialised role to meet a community need, the relevant specialists will complain and denigrate them.

FYI it's limited by doctors lobbying. Residency programs are funded by Congress, and the amount of spending was capped in 1997 and only raised once since, in 2021. And even the doctors are saying it's too low now.