Pretty much illegal everywhere in the US except for a few weird outliers. I think there’s one southern state that lets you have a bond instead of insurance?
When you're poor and you live in an area completely unserved by public transit and you lose your license because you can't afford to pay parking tickets, are you really going to stop driving and lose your job and become homeless?
We have statistics to show what unlicensed and uninsured driver crash and fatality rates are like and they're a lot higher than the rest of the cohort, but there's still a sizable part of the US population that does all of these things and still uses the same public road infrastructure as everyone else, often out of lack of alternatives.
If you never get pulled over, or you know some tricks, you slide by.
And to get your car registered in most states, you usually only have to pass an emissions test, have a valid license, and have proof of insurance at the time that you register the car.
This means that 11 out of 12 months, you get to drive around without insurance.
I think you can do this in most states; I know you can in my state (MA).
The most common is "farm implement operated incidentally over a highway".