MMOs aren't filled entirely with the bad people. It's just like the internet. Sure there's people on there who make it notoriously bad, but it isn't all bad, and mostly it's just decent people trying to have fun. MMOs are the same way.
MMOs are huge explorable worlds with ongoing stories that are continually updated. For SPRPGS, they're usually not huge worlds, and only occassionally updated these days via downloadable content that adds very little to the game. The persistent online world gives you more reason to attach to it, since it'll be around for a long time to come.
Different people play for different reasons. I'm a chatterbox, so I basically play MMOs to have a pretty place to blab in chat channels and make friends. It's fun to just lose yourself in a virtual world with others, forgetting all about the real world. Your immersion isn't trashed because you have other people in the setting too, people who are far more interactive than random NPCs.
"Sir,we are beginning our search for these rumored 'underwater villages'.They will not stay hidden for long". -Battle Droid Commander OOM-9!