part of the hook from MMO`s is you meet people online also playing them and to a degree thats what keeps you going back, if these games were solo player they just would not hold most people for anywhere near as long, If Wow for example was a solo game It would have probably held my interest a few weeks.

Although for me MMO`s have always been a platform to go roleplay with others, rather than play the games to get the epic gear etc which was much more of an also do to me.