I think the simple answer is just that they simply set the shows in the places where they produced them (LA, NY, or generic metropolitan area). Because it's a hell of a lot quicker and cheaper to film scenery at the immediate location than to go outside those boundaries or build sets for everything.

Though nowadays, with how incestuous Hollywood has become, you pretty much need to be born into show business to have any part in it. Which means they're taking people who are already inside the fold, rather than drawing in talent from all over the country. So everything just becomes about entitled assholes living in big cities.