Medical insurance companies in the US are the agents by which health costs are kept down. In centralized government health systems the government itself does this; in our system the insurance companies do it. They have the negotiating power that individual patients do not.

Part of keeping health costs down is rejecting some treatments as too expensive. Without this, costs will balloon forever as there is no one to say "enough".

There is literally an entire cottage industry full of people whose entire livelihood depends on getting people medical treatment after it's been denied.