Being able to do anything in a game wouldn't be the perfect game. You can write any story into an empty notebook, does that make it the perfect book? I'll just point at the ZP review of LBP: People buy games for the professionally designed content inside, not the user generated content. Most people have no clue how to design a game and many games don't lend themselves to being much fun if you designed them yourself (you'd have to know the solutions to your own puzzles after all).