Fundamentally, lawyers. As America's ruling class needs multiple massive cash piles to drain. All types of insurance directly or indirectly providing them.

Next, the insurance companies.

Last, the medical system owners (various) to include Big Pharma but also the other niches.

The better but related question is why it is such a shit show. The answer has always been that insurance was introduced into it. If medical treatment had been kept tied to market forces, then what people could generally afford would have continued to set pricing. As well as kept pathological industry motivations (harm) in check.