OK, lets talk about SE's only other MMO Final Fantasy XI. They didn't force full MSQ completion on you. Instead of comparing single-player games to an MMO, perhaps try comparing two things in the same genre.
If the story is the main show of XIV, what on earth do you do after it's all completed? Sit there idly twirling your thumbs? There is A LOT of other content outside of the Story (which really only lasts a short bit of time). MMOs pull players with varied interests, some will like story, some are here for other things. And there is nothing wrong with that.