Everyone kind of drags their feet on the answer but really the reason we spend so much is because we're still the only country that lets profit be the motivating factor in our health care system. That's the simple answer, and the most direct really. I always hear people hemming and hawing over what causes our health care to eat up so much of GDP with so little in return, but really it's because when you go to the doctor, even for simple things, there are like 12 entities which all want to make massive profits before you're done.
Also, I think that statement about how if everyone had top-notch care we'd go bankrupt is really, really misleading. And part of what makes that misleading is that poor-people and most people who can't afford care still get it. And when they can't pay up, everyone who wants money just turns to the government to pay for it anyway. I'd actually be willing to bet that if everyone had equal access to top-notch care, our percentage of healthcare spent as a part of GDP would go down. But the people making huge profits off of your care would lose out.
And that's what it boils down to. Maybe for people at the bottom it can be an ideological war about big vs small government or health care as a right. But for the people at the top, it's about whether private interest gets to make money or not.