Grega, people love MMOs because they ARE MMOs. It has more to do with the MMO part than the RPG, FPS, or any other part. MMOs are social games, people enjoy it because they like playing with others. The things I experienced in my very first MMO had the same dynamic nature that life has. Grinding and all are just are just a reason for you to stay in the game longer.
Take my experiences for example. In my first MMO game, I made a very dear friend who I used to spend most of my time with, we would talk so much during all our questing. Then there is all the random stuff happening. There was this lady who wanted food for her pet. (I found out that her entire guild consists of her family, 3 generations of it) I had some kind of hay which was a pet food, and I wanted to give it to her. But being an ultimate Nub, I dropped in on the ground for her while she was expecting a trade request. All of a sudden this other fox race person comes up, picks up the food and runs like crazy. I had promised her the food, so I did the only thing that I could do, chase the fox. Thus began an epic chase across the entire map. The fox had extra speed, but I had my flying sword. All the while chasing her I would keep asking the fox to give it back. And boy would she or he taunt me. In the end, I convinced the fox to give it to me, only to take it back and get told that her pet only eats meat, instead of hay that I had. >.>
I had a bare-knuckled fight with a Tiger dude because he kept hitting on my friend. We went on a Pearl diving expedition only to get eaten by some huge sea monster. I flew lady all the way across a map because she didn't have her flying sword. There are many many more stories that will stay with me, of all the good times I had with so many different people.
No single player game can give you that. Because its not the game, its the people that play it.