I think that there are certainly some issues with an artificial scarcity of doctors as well as overly restrictive regulations requiring MDs for certain procedures that don't require MD training, but the evidence shows that this is not the main driver of high costs. Physician salaries are only about 8% of healthcare spending[1] and according to my own research, only about 20% is due to all healthcare worker salaries (including physicians, nurses, physician assistants, etc. The remaining 80% is medicine, medical supplies, capital costs (MRI machines, hospitals) and administrative costs. Unfortunately there's no single cost that could instantly solve everything if eliminated; there's just a lot of middlemen each taking a tiny portion that add up to a huge total cost.
There are plenty of ways it could be made cheaper, hugely cheaper, but the regulatory system protects the established players / methods and prevents competition.
[1]https://www.npr.org/sections/money/2019/03/12/702500408/are-...