Freak accidents like just getting hit by a car while minding your own business are uncommon. Most accidents that happen are basically "accidents waiting to happen". If you are hit by a car and seriously injured (as in quadraplegic), your medical insurance in the US will still fall far short of covering the costs. With or without insurance, you are far better off doing everything in your power to avoid ending up in such a situation: Don't drink and drive, don't get in the car with an unsafe driver, and so on.

If you are concerned about getting cancer, you can get a very affordable cancer policy for a lot less money than major medical coverage costs. As with the above situation, the expenses involved if you get cancer are likely to far exceed what insurance covers. And cancer is influenced by lifestyle. Most Americans aren't terribly healthy. For example, obesity is rampant in the US. So I would be interested in a citation for your assertion that "millions" of people with "healthy lifestyles" get cancer.

Thanks.