There aren't any insurance companies that will even write you a car insurance policy that will cover the potential costs for a very serious injury. It is too risky for them. Heck, the maximum coverage you can even voluntarily buy in the US is lower than the minimum requirements in some other countries. Some countries require millions or tens of millions in coverage, some require unlimited liability. In most of the US many insurers won't even let you voluntarily buy more than $250k/person, and are only required a tenth of that.
(Seriously, it's quite magical. A high-limit umbrella policy plus a minimum-limit homeowners' or renters' policy and minimul-limit auto policy is generally less expensive than moderate-limit policies without the umbrella.)
The reality is, in many parts of the US, half or more of drivers have minimum coverage or zero coverage.