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.