Can you imagine how small the Lego section of a toy shop would be if they sold generic sets? There'd be six or seven sets of varying size, perhaps with a few accessory packs (wheels, people, gears). And next year... The exact same sets would be sitting in exactly the same spot. By marketing hundreds of sets as specific creations, the company can pad shelf space and (more importantly) give my kid something to choose between when shopping for a friend's birthday.
In the end, all the parts end up in a huge bucket in the closet that's used to create weird and wonderful space ships, bizarre marble mazes and all manner of buildings.