Like I said, residency training is the bottleneck. Part of that is due to the fact that CMS funding for post-graduate medical education has remained flat for the past ten years. There's also the issue that you can't just open a new residency program the same way you'd open up a McDonalds franchise. It takes years of preparation, investment, and recruiting of qualified personnel, and if a hospital isn't going to get subsidies from CMS then they're not going to open a residency program unless they're damned sure it can pay for itself.
One area where doctors are trying to limit competition is with restrictions on mid-level practitioners (NPs, PAs, CRNAs in the case of anesthesia.) While we do need to increase the supply of doctors we also need to take a rational look at what kind of care can be delivered by NPs/PAs rather than MDs. Sure their training is nowhere near as thorough, but you don't need four years of med school plus three or more years of residency to treat strep throat, ankle sprains and other minor issieus, or perform physicals, pap smears, etc.