I'd say it's the way healthcare and insurance work but I'm no expert.
Part of the problem is that it's almost always employer-provided in the US. That means that employers have historically paid most of the cost and consumers are just recently starting to see the costs. BTW I think this started after (or during?) WWII when employers were forbidden from raising wages to attract employees. To set themselves apart they offered health insurance as an employee benefit because that's all the regulators allowed them to do.
When you don't pay for something you don't pay much attention to it. So people don't make good decisions about how to use it. If you zoom out a little, people make bad lifestyle decisions for the same reason. If you knew you had to (visibly) pay a greater portion of your healthcare costs would you smoke, overeat, or regularly booze?
Stolen from here (not that I know who this guy is or agree with anything else they say, I just googled "healthcare if groceries were free" so I could copy/paste):
"He asked the crowd to imagine
[Download Audio: "The 'Free' Mindset" - 486kb] "if
groceries were covered by your employer; if you had grocery insurance.
So then when you go to the grocery store, you know, it's already paid
for, so you would insist on caviar and Alaskan salmon... And if you
were told, 'No, no, no, you can't have caviar," you'd say, "Oh, yes, I
deserve caviar just like everyone else does.' And you would quit asking
about the price,
because it's already paid for. And you would buy more than you need,
because after all, if you don't use it, well, we'll just throw it away,
it's already paid for. You know what would happen over time?
Literally, within three or four months, supermarkets would quit putting
prices on the products because the consumer wouldn't care about the
price. And if I were running the supermarket, I can tell you what I'd
do to the price. I'd raise it quite quickly 'cause, after all, no one
cares what the price is. And the consumption would grow rapidly and we
would have in the grocery industry the same situation we have in health
care.""