I suppose it's like playing Caesar or SimCity or one of those city building games. When you start out, especially as a GM, you have all these ideas: maps, mountains, exotic words, magic system, bloody history etc. Then when you put it all down neatly, create that perfect world and wait for those interested to put up their CS, you realize that you are spent. Suddenly, there are other people - wanting to write with you. Horror of horrors! You quit, say you have a life, and go on to the next world. Only this time, it will be grander! Bigger! Muahahaha!

Repeat ad infinitum.