But on the other hand they don't, because true RP is generally far too pretentious. Even in movies/TV, you see it mainly only in villains and historical dramas, or for comedic effect.
Most people are generally referring to a kind of middle-class London accent. Like Jude Law for example. Neither "posh" nor "regional" nor "lower class" nor "multicultural" (quotes for irony where necessary, obviously London is a region). I'm not sure what the name for that is though?
As a North American, I find it one of the harder accents to understand. It has undergone a lot of interesting sound changes in recent decades, not shared by many other dialects. Likely a result of dialect flattening from 20th century population movements. "Football" -> [ˈfʊʔbɔo] (sounds like "fuh-baoh" to me) or "bottle" -> ['bɒʔo] {"bah-oh"?) or "Tuesday" -> [tʃuːz.deɪ] ("choose day").
"The water in Maiorca doesn't taste like what it ought to"
"THE WARTER IN MAJORKER DUNT TASTE LOIK WORRIT ORTER"
You really think so? Maybe it's just me, but I can't imagine how anyone could mistake the two. (Possibly different people have different ears for accents.)
My favorite is Margot Robbie's over-the-top "Brooklyn" accent for her Harley Quinn character.
It opens up American roles for them. Nobody is going to cast Brad Pitt as a Brit, an Australian, or a Kiwi. He's a fantastic actor, but they're going to use the homegrown talent. So there's no incentive for American actors to really learn another accent. Instead, we get a mountain of non-American actors doing mostly-good-but-sometimes-awful accents because that's where the money is.
Except for Irish roles, for some reason.
Brad Pitt as a northern Irishman: https://www.youtube.com/watch?v=2FjQQrxBhwQT
Brad Pitt as an Irish Traveller: https://youtu.be/9Jar2XkBboo?t=28
FTFY