The theory behind Medicare Advantage is that it would cost the government less than traditional Medicare because the private insurer would be more efficient. Guess what happened.

I think the logic of running a more efficient company is true - they are making more money operating them than the government can/is.

The insurers are such behemoths and so largely vertically integrated it is controlling the system instead of improving it.

Notice how there is rarely ever any new competition in the health insurance space to drive down pricing.