I imagine the same reason so many Americans think of the UK as just London, or France as just Paris. I don't know what that reason is, but I imagine it's probably the same reason.
Honestly I can't get Wiltshire out of my head. Rhotal rs? What? They sound like they're from New England until they say something that makes my brain twist. I love it.
That's not true America is Texas, Florida (if it involves alligators or trans-hatred), California, Las Vegas and New York. Everything else is irrelevant.