Even if financial constraints were removed, the notion that we could produce twice as many physicians is ludicrous. There simply aren't enough attendings able to supervise a lot more residents, nor are there enough patient cases coming through the teaching hospitals to give that many residents the practical training they would need.
It's the AMA.
"Free to add more" is hiding a LOT of important nuts and bolts of the guild system that discourages new entrants.
Can you imagine if that website were for programmers?
"Waaaah we need more Staff Engineers at OpenAI but we just can't afford training them due to Congress not doling out enough for it, boo hoo".
State medical boards are the entities which ultimately decide on physician licensing criteria.
This AMA/ACGME protection theory is covered by Yascha Mounk at https://www.persuasion.community/p/deaton and elsewhere.
Discussion of physician protectionism is also found in the 2017 book by John Hopkins political science professor Steven Teles and Niskanen Center vice president Brink Lindsey The Captured Economy: https://global.oup.com/academic/product/the-captured-economy...
The New York Times covered the AMA's lobbying in this 1986 article: https://www.nytimes.com/1986/06/14/us/ama-board-studies-ways...
There's also the 1970 article by University of Chicago economics professor Reuben A. Kessel, The A.M.A. and the Supply of Physicians: https://scholarship.law.duke.edu/lcp/vol35/iss2/4/
There are valid reasons to criticize the AMA but blocking the training of new physicians isn't one of them.
The AMA, and physicians in general, are obviously not the primary obstacle here. Complaining about their past actions won't solve any of the actual problems we have today
What we need is competitive markets, what we get is heavy intervention. For example, https://en.wikipedia.org/wiki/Certificate_of_need system means that to build a new hospital one needs to go to their direct competitors to ask for their permission. Imagine if to open a pastry shop you needed a permission from every other pastry shop in the city - bonkers.
The USA doesn't recognize those other doctors (in more ways than 1 apparently) and that is an aspect of the issue.
Prestige and potential higher pay for specialisations will still attract people to the profession, although maybe not if the university costs for medicine remain as high as they currently are.