https://randomcriticalanalysis.com/why-conventional-wisdom-o...
TL;DR: As people/countries get richer, a larger share of their money goes towards consumption. It's not just that Americans pay more for the same procedures (sometimes they do, sometimes it's just sticker prices) but we consume more healthcare the more money we make. So it skews costs up disproportionally. That wealth also enables chronic health and lifestyle problems that are expensive in their own right.
I'm not sure I'd buy the theory entirely, but it's very well argued and as a null hypothesis it makes a lot of sense.