I don't even know why outfits / guilds / etc. are such a big deal in MMOs. There's nothing there! It's just a menu with some people marked as leaders and some marked as members. Yet it's surrounded by unimaginably stupid drama constantly.
1. Some people like stupid drama. It seems hard to believe, but that's an upside for some people.
2. MMO creators LOVE guilds because when you join one you feel an obligation to help them out, keep playing with them, etc, so even if you personally get bored of the game you might keep playing (and paying) just because you don't want to let your buddies down.
3. It's become expected, you play an MMO you join a guild... it's just what people do regardless of if they really want to be in one or not... You read forums or whatever and it's just a constant refrain from EVERYONE "join a guild join a guild find a good guild guild guild guild" so everyone just feels like that's something they have to do.
4. Spam invite guilds will constantly bombard you if you don't have one. Many people probably don't realize you can auto-decline guild invites (in most mmos anyway) so they just accept one to make the bombardment stop.
5. Guild leaders (and officers to a lesser degree) get to feel like big shots who are in charge, even if their guild is full of 100's of people who they randomly spam invited and who don't listen to them ever... they still feel important so they love guilds.
6. E-peen value. If you're in a guild that accomplishes something you get to take credit and feel better then all the peons even if you were on vacation that week and didn't help in any way.