A core reason I think this is the case here is that the US insurers are not very "free-market": they are highly regulated & essentially oligopolistic. Their premium pricing bands are bounded by the Gov't who mandate that premiums don't exceed a certain % of what they actually spend on care. So, if they pay more for care, they can actually charge the customer more. This is like a contractor who can charge a % based on project size.
Of course in a "free market" system, competition based on premium price should counteract this. However, due to another complex set of factors, this pressure does not seem to be very effective. (This includes the US system being employer based, and again the relatively oligopolistic structure of insurers). That said, I'm still very surprised there's not more competitive pricing pressure in the US, despite what seems to be some strong potential corrective forces, like employers self-insuring to reduce their costs.
(setr's comment about the "retail" price not being the real one is true, but it's also true that insurers still pay more in reality in the US vs. other Countries).
[not a pro on the regulations, so please correct me]