Even when it's not the insurer, it's at least a hospital. Many a doctor around me that used to have a private practice sold to one of the hospital chains, as they promised more money than by owning, solely due to superior collective action advantages. A large insurer can bully a private practice into cutting costs, but a hospital network that handles 40% of ERs in the metro area? The insurance company can lose. So everyone makes more money but the people paying insurance.

On top of that the ACA prevents new physician owned hospitals from being established and placed restrictions on expansions of existing ones

Are you talking about Certificates of Need? Those have been around for a lot longer than the ACA [0]

[0] https://en.wikipedia.org/wiki/Certificate_of_need#History

To be fair, this is because there's long-standing [but disputed] evidence that healthcare providers drive up costs/utilization when they can refer to hospitals they have equity stakes in.

Messy business!

Not quite true. If you own the providers, getting people to pay deductibles and copays (i.e. getting treated) will yield way more money than just having them pay premiums.

But undercutting the other insurers/providers should earn you more, if it were competitive, which I guess it's just not

Correct, which is another reason why they are buying the healthcare providers. It allows the insurers (“pay-viders”) to strong arm independent doctors and smaller insurers out of the market.

Ok, I get it now. No other explanation has made sense so far, it's a cartel.

The insurers are legally obligated to pay out 80-95% of their premiums for treatment. So the only way to grow profits is to spend 2x and much and charge 2x as much. Sure you only make the same 5-20% margin, but it's on 2x the revenue so it's 2x the gross profits.

Uh, no… they want to deny claims. The best situation for insurers is that you are healthy for a while, then abruptly die of something that cannot be treated.

They are accepting claims that are way more expensive than in any other country, so I don't think this is their game

Uh no, they don't. Not if they're also the ones who provide healthcare. Simply denying claims isn't even remotely close to the financially (and obviously not the politically) optimal strategy.

The optimal strategy if you own both the insurer and the provider is a combination of premiums, copays, deductibles, and maybe even some totally unnecessary care to drive up volume.

Lower margin on dramatically higher volume is still dramatically more money. Lower margin actually provides political cover for your $400 billion revenue years.