Well this is rather obvious. There have been a number of attempts at MMO non-games that were solely for player interaction--but I've never seen one actually work. People play games to have fun, or at least to interact with others to do something that they wouldn't be doing in real life.
Nobody is going to bother installing a multi-gigabyte glorified chatroom.
Actually, this is what lots of MMOs end up being anyhow. Being a leader of an alliance in Eve or a merchant, the most used tools are TeamSpeak and the chat rooms. Lots of tabletop Role Playing Games also end up being social gatherings where the primary enjoyment is the resulting laughter, and the game was just a pretext.
Which is exactly the point. Without the pretext of the game, less people would show up for the gathering, resulting in a lower "resulting laughter" quotient.
As one of the "shy ones", I can confirm that having a shared activity encourages me to come to the event, and ensures that noone is "left out" (as their character has something to do, even if they personally have nothing to add socially). An interaction within the confines of the 'game' encourages more voluntary interactions, not to mention giving people a pretext for conversation, etc.
They are not all flops. There are some MMO social environments that are huge, especially outside the US. Kids like it. Habbo.com for instance has many millions of users, mostly from Finland and the UK.
Nobody is going to bother installing a multi-gigabyte glorified chatroom.