Why don't they create a game engine that allows console gamers to create their own game from it?

...Sort of like The Sims, yet on a more grand scale. What i mean is, they could design a game engine with a level editor / character studio, that allows the gamer to create their own game.

For instance, a games company could have an online store which allows the gamer to purchase and download costumes / weapons, animations, etc, compatible with the game engine / level editor. That way they could create their own game -- something everyone wants to do.

Not only would a company make a fortune from the sale of the software itself, but also from the sales of downloadable meshes / animations.

Surely this is a possibility. What do you think?