MA plans negotiate with providers, similar to a regular commercial insurance company, for different “deals.” As you may be aware from commercial insurance, that typically means higher rates, since everyone along the line has to get paid across the providers and the insurance companies. Much higher profit motive than a socialized good.
On top of that, MA plans have an incentive to deny coverage wherever possible, because as a for-profit entity they want to make as much money as possible, not get old people healthier.
Young people have trouble fighting insurers on denials. I know this firsthand. But it’s doable, because we tend to have more energy and less cognitive impairment; I’m generalizing here, but dementia is a thing that happens to a lot of people in old age, for example.
For older people, fighting insurance companies is just about impossible. Often, this falls on their younger family members to help (ask me how I know, lol), but that implies their family or friends are willing to help. Not everyone has that.
And it shouldn’t be that way.
[0] There is a thing called Excess Charges that providers can charge, if they don’t accept the Medicare fee table, but still accept Medicare generally. They have to tell you before the service, or you are not responsible. It is also capped at 15% of the service, and is illegal in some states entirely, and capped lower in others. Some Medigap plans also cover excess charges, but they are on the more expensive end.