On the contrary, uninsured motorist insurance in the US is very inexpensive. $76 per year for half a million dollars coverage is typical (current quote from Geico).
Except that the very idea is astonishing when compared to the legal minimum insurance which sits at around 1 million dollars in the UK iirc.
I suspect that the main difference is that we are happier conceptually to say "you cannot afford insurance = you cannot drive". Driving without insurance is, of course, a crime. People still do it, and insurers will cover accidents involving the uninsured - but because it's less common the feature is basically just included by defualt in most fully comp policies
It's easy to exaggerate the differences between the UK and the US, merely because the US is a bit unfamiliar. When there is a small cost for something that exists in both countries, it is not an earth shattering difference for it to be paid from taxes in the one case and part of the insurance premium in the other.
To wit, uninsured motorist coverage is provided by the government in the UK, from what I read:
"The Motor Insurers' Bureau (MIB) compensates the victims of road accidents caused by uninsured and untraced motorists."
You can get $1 million in liability insurance in the US, and people do. It's not unaffordable, and while it is a pre-requisite to the uninsured coverage mentioned previously, it can be purchased alone.
I suspect that the main difference is that we are happier conceptually to say "you cannot afford insurance = you cannot drive". Driving without insurance is, of course, a crime. People still do it, and insurers will cover accidents involving the uninsured - but because it's less common the feature is basically just included by defualt in most fully comp policies