Insurance is one of the oldest industries in the US, which means they have accumulated many (probably well meaning, but ultimately stupid) regulations over the years.
For example, to be taxed as an insurance company (e.g. avoid paying tax on premiums that will partially be paid out in the future), most states assign a maximum profit margin.
That means, in order to make more money as an insurance company, it's perversely in their best interests to increase the cost of medical care as much as possible. Medical care is not like cat bonds that might wipe out your risk pool, so it's just a cashflow game.
That's not to say there isn't any downward pressure. On a day to day basis, individual claimants eat more into that margin than was budgeted, so a company might have to raise rates higher than their competitors (essentially they have some pressure to deny claims), but there is very little systematic pressure to reduce overall costs. Also, marketing budget does not count towards profit, which is why insurance companies are in this rat race of brand advertising to steal customers from each other. With auto insurance, for example, upwards of 40% of your premium is just going out the door as ad spend.
Anyway, I would assign a lot of blame in this perverse incentive scheme to poor regulations.