A limited number of physicians, especially general practices? High cost therapies and treatments? Capital-intensive diagnostic devices? Lack of access outside of cities?
There are certainly a lot of things that could be done better, but this isn't 1950 -- some available medical technology is incredibly expensive.
Simply unshackling Medicare/aid and allowing it more freedom to negotiate costs would go a long way towards reducing US health care expenditures.
I.e. doing for more payments what the Inflation Reduction Act allowed Medicare to do for (a limited number of) drugs: https://www.cms.gov/files/document/fact-sheet-medicare-selec...