Competition. Hold hands and let's be nice doesn't cut it, someone will have to start being competitive to others. Then it's beat each other up until they're exhausted. Add some smack talk and play around to enjoy each other's company.
Same reason people enjoy movies or books that aren't necessarily 'fun', I imagine. Being told a story, being engrossed in a character or setting, being made to think... we enjoy these things, obviously, but they aren't typically what we think of when we think of as fun. Yet, they're still totally viable reasons for playing a game, I'd think.
... of course, not much of that really applies to SC. That's just fun. :3