Because insurance companies add nothing; they just rake off the top of the betting pool.
Insurance is simply a bet by the purchaser something will happen, and a bet by the seller it will not. The seller, like any bookie, tries to set the odds so that what he has to pay out in total losses is less than what he gets to keep on total wins. When the bookie is selling payments to doctors, he no more adds a value of health care than a bookie selling payments if the Packers win provides points on the board.
A company manufacturing medical equipment, such as pace-makers or artificial joints, say, or a laboratory providing diagnostic tests, provides something of value, something which can actually further health of a patient, so profit to them is not a dead loss to the system. It may be exorbitant in some instances, but that is a different matter: profit to such bodies is a proper enticement towards adding real value.
But insurers simply stand between a mass of patients and potential patients, and persons who actually provide health care, and short-stop as much as they can manage into their own pockets of the total outlay made by the former for the services of the latter. In the course of this, they very frequently engage in sharp practice, and welsh on their obligations to pay, denying services or even cancelling policies when they have lost their bet with a policy-holder.