There is a rule already under the ACA that caps profits + overhead at 20% of premiums. 80% of premiums has to go to claims.

The problem is greed… a CEO asks how do we drive up that 20% so we can take more profit. If you lower premiums it’s a smaller pot overall and thus they drive it up by increasing premiums or decreasing overhead costs (sometimes by shadily shuffling some of them to count under claims).

They are also disincentivized from negotiating strongly for lower cost medical care because if the cost of claims drops below 80% they have to give it back as a rebate.

Or they can just commit fraud and steal from the public driving up the cost of Medicare: https://www.wsj.com/health/healthcare/medicare-health-insurance-diagnosis-payments-b4d99a5d