Not just healthcare, the US outspends everyone at everything: infrastructure, education, defense, research, police, housing, prisons, etc. both in relative(by % GDP) and in absolute numbers.

It does so because it can. It's the wealthiest country on Earth so such high inefficiencies where a lot of the money is squandered to the benefit of few wealthy and unscrupulous parties, can be financially absorbed while still delivering a system that's functional enough for its citizens to not revolt over and want to hang someone. After all, it's still better than scary communism.