There are all sorts of criticisms one can level at the US Health "system" (as well as the UK NHS), but it fundamentally has to come down to efficient use of resources. In the UK, we pay money to the NHS (via tax) and the NHS spends it on our healthcare, in the form of people's wages and equipment costs etc. Internally, the NHS is set up to get the best healthcare result overall for the entire country, with the money it has.
In the US, there's an extra party - the insurance company. You pay your money to them, and then they pay some of that to the hospitals etc. for your healthcare. The key is some of - they're a business, with costs to pay, and profits to extract. You're making less efficient use of the money overall.
Capitalism, when used properly, is a great way of getting efficiency. For consumer goods lower costs means lower prices so more sales, so more money. Capitalism working perfectly, so the consumer is paying as little as possible. But the insurance companies aren't incentivised to produce efficient healthcare, the very opposite. They're not evil (mostly), that's just how the market is.
* Possibly still my favourite quote from any newspaper ever, was a sadly ill-informed "Investor's Business Daily" trying to be rude about the NHS, saying "if Stephen Hawking were British, he's be dead". Stephen Hawking IS British (I've seen him in Cambridge), and has been well cared for by the NHS to live to the ripe old age of 70 - very rare for someone with Motor Neurone Disease.