This is probably one of the biggest cultural differences between the US and the UK, not only here is driving without insurance a criminal offence, over here we have (had?) a much greater level of functioning public transport so there is not quite the immediate need to get a licence, and the insurance would likely be prohibitively expensive if you did.
As to why it's happening in the US? I don't know, I don't live there, there's probably some proportion of the population so clueless that think if they don't drive it'll save the planet and prevent Colorado from being underwater 100 years from now, but outside of that? Other than expense, no idea