Sure, but the average American also gets paid $20k more than the average Brit, on average.
~25% of the federal budget goes to medicare and medicaid, i.e. healthcare for other people. On top of that, you're paying for your own medical insurance as an implicit deduction on your salary for your employer-sponsored healthcare plan, or you just pay for your plan directly if you're self-employed.
Those percentages add up.
Whereas in the UK, or in Sweden where I'm from, you only pay once through your taxes for healthcare for everyone, including yourself.
On top of that, copays are higher in the US, annual deductibles are much higher, procedures are much more expensive, medication is much more expensive. Healthcare in the US is simply disproportionally more expensive than in the rest of the world, as a percentage of people's income, and as a percentage of GDP. It's got nothing to do with salary levels.
Edit: everything we pay out of pocket we can deduct from our taxable income (it's an HSA plan), but still.
I'm one of those 90%. My health insurance (family of 4) costs more than my house payment, and the annual deductible is over $6000 (for one person). Either the premiums or my deductible goes up every year. In terms of total cost (monthly premiums plus annual deductible) it's also pretty much the least expensive plan that I can get.
It's not that health care here is bad, it's that it's ridiculously expensive compared to most other places in the world.