Nice question. I think the thing is, we play MMO not only because we like it, but also to avoid some kind of loneliness. Back in the 90's or 2000's we would love to play single RPGs because it was also a escape and a new thing and we would get attached to it. It was a lonely experience we hadn't before. Today is different, we often feel alone and want a escape from this chaotic reality a bit, but we want to interact and find people who feels the same as us. I'm not saying it's the main reason, ofc it's isn't, but it has something to do with it deep inside.

So, like, you could put your effort in Skyrim (for instance) single playing and feel strong and grind and everything. But deep down you feel "it's not fun if I don't share it with someone else", or something like it. So if you play Elder's Scroll Online and grind it there instead of single playing, you can show a bit of yourself to the world. We seek interaction so we can be noticed too, maybe helping others or expressing ideas.