When something is paid for from a big nebulous ball of money rather than straight out of people's pockets, the downward pressure on prices just isn't there in the same way. The conversations between practitioners and insurers are about whether something is necessary, not about whether or not the practitioner is charging too much for it.
Here in the UK we see it, too - not so much in human healthcare since we have the NHS - but very definitely in animal healthcare; vets' bills have skyrocketed over the last couple of decades, in a mutually-reinforcing feedback loop with the rise in pet health insurance.
Kind of amazing how perfectly this illustrates that in the exact same economy, the system with single payer mostly works to keep prices sane, and the private insurance model goes off the rails
This is not some spurious speculation. That market-based systems drive down costs enormously is replicated across dozens upon dozens of industries. It's one of the most replicable results in economics to the extent that economics can be replicable. As for why the costs in other countries are not quite as high as the U.S., it's because health care costs also increase as per capita GDP increases and the U.S. has higher per capita GDP. Moreover, because the U.S. has some aspects of its health care system still living more in the private sector, there is less top-down rationing. Other countries see very clear examples of rationing, so people spend less on end-of-life care.
That actually sounds a lot like speculation. You're claiming that the most largest structural difference in the healthcare system in the US compared to other countries is unrelated to the difference in costs, and that other factors explain it, based on inferences from things not related to healthcare. I don't understand how you can have any degree of confidence that single payer versus private insurance has no effect at all based on what you're saying.
As for the difference between the U.S. healthcare system and healthcare systems in other developed countries, if they had the same output of health care services, I would agree, but the US has far more diagnostic equipment per capita and more advanced treatments. It has the best neonatal facilities in the world. These are extremely costly and some of this expenditure doesn't actually produce that big of a difference in outcomes because it's really just dealing with end of life care, or a relatively small number of patients (e.g. premature infants). But when people are free to spend their money as they wish, this is what they prioritize. I'm also not ruling out that the U.S. healthcare system has less efficient government intervention than the healthcare systems of other developed countries. This is not mutually exclusive with the idea that, more broadly speaking, the structural change towards more government intervention has raised costs across developed world healthcare systems.
The more important question in my opinion is why health care spending across the entire developed world has skyrocketed over the last 40 years. It can be fairly inferred that the cost increase is directly related to increased administration and a lack of the kind of cost-cutting innovation seen in every industry that is substantially more market-based / consumer-driven, i.e. less top-down regimented, than healthcare.
Even cosmetic and laser eye surgery has seen costs come down over the last 40 years. That's what we should be seeing in the rest of the healthcare industry. And the reason we're not is because they operate under very different economic forces than cosmetic and laser eye surgeries, on account of government encouraging people to be covered by either public insurance or private insurance.