The US is (exaggerating a bit) a nation of obese, sedentary substance abusers. We are sicker on average than other developed countries and thus have a higher demand for healthcare.

We might be able to eke out some minor improvements by tweaking the payment model and eliminating some waste. Those things are worth doing, but they won't fix the fundamental problem. The US won't get healthcare spending down to Japan's levels until Americans start acting like Japanese.

There are some other key factors as well. A large fraction of healthcare spending goes toward treating elderly patients with serious chronic conditions in their last few years of life. Some countries explicitly deny care to such patients because they don't think it's justified on a QALY basis, but Americans seem uncomfortable with rationing on that basis.

And some aspects of the US healthcare system are top notch. For many types of cancers we have the world's best 5-year survival rates. There is a thriving medical tourism business where patients from countries with socialized medicine such as Canada come here to receive rapid treatment instead of waiting for years for something like a hip replacement.