1. So, what's up with accents in mainstream American media? Pretty much everyone in TV and the movies is forced to speak with the same regional dialect.
So here are American accents that I never seen on TV: the local dialect in Oregon, the accent my Illinois relatives have, the accent my Tennesee relatives have, and the Boston accent my old boss had. There are probably several Southern accents which you can add to that list. Sometimes you hear a Canadian accent, but most Canadian actors are forced to speak like Americans while filming.
I suppose this tendency to only have one "official" dialect isn't limited to America, though. Ancient Greek had four different dialects, but Attic (i.e. Athenian) was the official, most common one. Every now and then, you find Athenian comedies which poke fun at the funny-talking Spartans. As for the Doric Greek dialect, I've never seen it get used.
2. If you've seen my Nancy Drew: Secret of Shadow Ranch walkthrough, you probably heard my southwestern accent. That's actually an Appalachian accent, which I took directly from Li'l Abner. It just sounds mildly Southwestern when I do it.
I might have mixed in a bit of Texan, for good measure. I don't remember, because I made those videos years ago.
3. The only accents I sometimes hear are New York accents (and occasionally other accents from that area). But I only hear them on shows produced in New York. I suspect that actors from New York, who have moved to Hollywood, get forced to drop their accents.
General Hospital has some native New Yorkers who don't bother hiding their accents. When I first started watching, it took me a while to understand them, especially the guy who mumbles all the dang time. I just wonder if he mumbles like that in real life, or if that's just his character.