The #1 reason is hospital-insurance price collusion. Because of the regulatory red tape, and because we depend wholly on 3rd party payers for all basic care, there is a ton of corruption that keeps the hospitals and insurers scratching each others' backs. It's extremely messed up. But it's *because* of government regulations and interventions that this has become the norm.
A second reason is the chicken-egg problem. We have more discretionary spending, so what do we spend it on? More health care. Americans go to the doc for every little thing. In single payer systems, they turn you away if you aren't really sick. For minor things, you have to get in line--a very long line that can last months. We spend on health care because we can. We waste tons of money on tiny problems, which, to me is fine--it's your choice if you want to spend hundreds of dollars getting that mole on your neck looked at. But know that, in single payer, you don't have that option.