The thing with Korean games, I think, is that they have an unabashedly monotonous and elongated grind. The way you play when you sign in is consistent, and the novelty comes from the playstyles of the other people you're with rather than from new circumstances. It's more like Counter-Strike, then. Which means it's very unsuitable for the more individualist Western audiences, who appreciate the value of a world populated by other living beings, but find it quite distasteful to depend on the competence of Random Internet Fuckheads to provide an interesting experience. (Or, at least, that's what I think). When you make an MMO for Westerners, you need to make your world varied and interesting, so that the value of the options available to a given avatar depend less on strangers and more on the avatar's history and location.
Being friends with people in the game completely invalidates all of that, but once you get 'em to that point, you're left with the people who already made that decision.