I don't think it's mandatory for people on tourist visas to come to the U.S. so some portion of visitors probably don't have it.
Huh maybe not - I thought being able to show you could cover expenses while there (so insurance) was part of the questions on entry or when submitting for the waiver.
I think it is for many countries but especially on longer stay visas rather than simple tourist visas.
That's just to check you're not planning to work on your tourist visa and generally means having enough to cover accomodation, transport, food, and entertainment.
Many don’t fully cover the USA due to the exorbitant costs.
Not if your "travel" is multiple months spent working there.
I already linked it on anothe rthread, but Germans can get insurance for that for up to 5 years, including the US tripples the price so, even if it remains quite affordable. So there are solutions I guess.