It's because it's challenging, and the challenges are shared. You can be any character you want to be in this reality, any gender. You can do magic, wear special outfits, dye your clothing, adopt special armor, accessories, etc. Outfits can communicate a lot about who you are and how sophisticated you are. You can spot the newbies, and the big bad dudes you can tell by how they're dressed, by what weapons and objects they're carrying.
Cultures vary, too.
All these people used to playing Doom and Quake came on and treated it like Doom and Quake , but others were trying to have interesting conversations. Some treat it like a traditional role-playing game, or like a renaissance game. In fact, a plurality of cultures is spinning off from Ultima . You can find 'em on highly interactive webpages all over the Net. I feel weird when I talk about playing the game.