I'll bite. First, each human is kind of a separate universe, another 80 billion neurons to converse with, each with our own histories and vastly different knowledge and experience. In a conversation, we learn a lot from each other, and better understand how we can be different in skills, and even in basic things like emotion, motivation, etc.., better understanding what it means to be a human, and better understanding what it means to be in general. Also, it's very important for us to maintain some kind of social contact (I think written counts as well), because our brains language ability will degrade and we will lose critical skills including reading social cues.

Speaking of social cues, interacting with others specially in a complex environment where there can be severe competition as well as cooperation and difficult coordination, is something that also is worth practicing.

I have nothing against solo games, but this kind of thing is not practiced in a solo game.

Finally, I think other kinds of games (e.g. in competitive games) tend to have very simple interactions and objectives, compared to an MMO: there's a clear objective to win that's shared by everyone. Some MMOs have much more interesting interactions, where each person is interested in a different thing, and I think this contributes to a very rich atmosphere that isn't just 'Go win, try to win match, go out', i.e. more life-analogue (without other limitations of life, like you can't actually die, and being poor isn't as terrible as it often is IRL :( ).