I could be wrong, but I think most MMO studios hire writers based on an audition period, not unlike the screenplay review process in Hollywood.
I don't really consider myself a writer although I do have to make a lot of narrative decisions; however, I appreciate the fact that both game design and narrative writing are not generally well understood methodologies. Most writers can't write great books/movies and most game designers can't design great games. I think as forms of entertainment media, the reasons are generally the same and that understanding one allows you to understand the other. And on my usual rant, most game designers don't understand player psychology and most writers don't understand reader psychology. Even when you do, it doesn't make the task easy--just easier to assess your progress. But that usually leads one to say, "This is crap; I have to redo it."
Now that I think of it, I don't think they generally have someone who fills the role of an editor on staff. They probably peer review, but the bottom line is that writing for an MMO generally involves a lot of fragmented, team-oriented scenario writing, where screenplays are generally unified under a single author who spends years writing one story.
I'm tired and drunk, so apologies if my musings here are sort of incoherent.
Ok, now we're going to get slash fiction of Wint x Kachi somehere... rule 34 and all...
Never confuse your inference as the listener for an implication of the speaker.
Good games are subjective like good food is subjective. You're not going to seriously tell me that there's not a psychological basis for why pizza is great and lutefisk is revolting. The thing about subjectivity is that, as subjects go, humans actually have a great deal in common.