Because I know of no better way to become cynical, burnt-out, and hateful than to go into a project expecting to change the world. The world is very big, and it doesn't give a damn about you. Unless you have extraordinary resources(I'd put the lower bound at "in control of a nontrivial country"), your probability of success is minuscule. Sure, it's fun to think about what would happen if you get lucky and achieve great fame and success, but it's like thinking about what you're going to do with your lottery winnings. Dream if you like, but you're a fool if you expect or demand it.