Even if I didn't want the multiplayer experience, I'd still want to play MMOs for the following reasons.
1. I always headcanon RP my game characters and MMOs are generally great at having NPCs which support that.
2. MMOs are persistent, living, games that get regular updates. If I buy an Elder Scrolls game, I can count on the base game plus a couple expansions before it stops being supported. As long as MMOs are running, they're always getting new content to do.
3. Even when my FC's quiet and I'm essentially soloing, I love seeing all the other players out doing their thing. Playing "alone among many" as I like to say makes me feel part of a vibrant and dynamic community. It's like real life; I have my friends and family for social activities, but there are always other people at the restaurant if we go out to eat. The same thing applies to the game. Single player games are fun for what they are, but they can feel lonely after a while whereas in an MMO there's always activity around me.
4. For XIV specifically, the story's so good that even if I never interacted with another player I'd want to play it.
5. Sometimes it's simply the genre. I prefer fantasy games to other genres and most MMOs fit that to a T.
6. Since XIV's so take-a-break friendly, a solo-friendly design lets people come in and out to play what they want when it's available. There were times when I wasn't able to put the time into an MMO, but I could sub for a month or so at expansion launch, again for a couple months mid-cycle, then again for a month or two before the next expansion. Since I was busy getting caught up on the story I was essentially playing solo, yet I enjoyed that time substantially.
THIS. I won't lie, I wish MMOs had the social aspects they did back in the early MMO to Vanilla WoW days, but that had as much to do with the times as it did game design. Even so, I think more solo-friendly design is better for the genre and the games themselves in the long run.