So, I don't disagree that there are inefficiencies with private payers, but I do disagree that they're significant, or the reason US costs are so high, or that insurers deny so many services. You can see this for yourself with Medicare's admin overhead. Admin overhead is, roughly, the ratio of money spent by insurers to money insurers pay to providers. Medicare has "low" admin overhead --- but that's in large part because they serve the most demanding segment of the market. If Medicare covered 30 year olds, their admin overhead would mathematically be significantly higher: same money in, much less money out.

I agree with you about the efficiency of HMOs, but customers hate HMOs.

A useful Google search: "National Health Expenditures by Type of Expenditure and Program: Calendar Year 2022". It's a single spreadsheet, and it's really something. It covers insurers (public and private), providers (hospitals and outpatient), facilities, state health care programs, even dental, all on one sheet. The numbers are hard to get around.