I think that's called a 'rohtic' pronunciation (never studied linguistic) and carried over into the American dialect from an older English dialect. see https:/

I think that's called a 'rohtic' pronunciation (never studied linguistic) and carried over into the American dialect from an older English dialect. see https://en.wikipedia.org/wiki/Rhoticity_in_English https://en.wikipedia.org/wiki/Rhoticity_in_English