Healthcare services (doctor salaries + facility overhead) is 80%+ of US healthcare spend. At first glance increasing supply of doctors seems reasonable. But how do you do that? Number of doctors (esp primary care and mental health) has been declining because of decreasing reimbursement / payment and increasing burnout. Being a doctor requires a decade or more of making no money, working long and unpredictable hours, and tremendous stress -- literally making life and death decisions. Can't just spin up supply overnight. To really increase supply you'd need to increase doctor salaries or decrease the cost of becoming a doctor. The former would not reduce costs, and the latter also is not ideal

One perhaps better way to increase supply is by rolling back consolidation. So same # of docs, but more independent practices. Another way is letting nurses / others practice at the top of their licenses, although I'll grant your point that physician societies sometimes make this hard. Also, one could argue that having more work done by low cost providers would encourage more volume and potentially unnecessary care

The main overarching issue though, even if it was as simple as increasing supply, the healthcare system in the US is not designed to optimize cost efficient care. It is designed to maximize profit and regulations arent optimized around this. So when hospitals view patients as numbers, the implication is that they will milk every last dollar out of a patient (30% of healthcare spend is unnecessary care). Insurance companies are incentivized to deny as much care as possible. No organization with any power in the system is incentivized to view and manage healthcare costs in an optimally cost efficient way