They are super expensive because
1) emergency rooms are required to treat everyone regardless of whether they can pay or not, so a lot of people use them for free non-emergency treatment. Medicaid is supposed to cover these people but Medicaid doesn't pay enough and hospitals have to recoup this expense somehow
2) hospitals CAN charge paying/insured people more because insurance pays most of it
3) insurance doesn't really care how much things cost because they will either just raise everyone's premium to cover it or they will find some excuse not to cover it