Medicare sets the rates government pays for providers. My doctor does not accept medicaid, ACA insurance and limits medicare to existing patients only.
My previous physician did the same before he closed his proactive.
My brother in law did the same before he closed his practice.
What part of government single payer do you not understand? Medicare may be called insurance however, government sets payment rates and if they will cover the service.
Physicians that accept medicare, medicaid have numerous NP's and PA's to see the cheap government clients.