Because the builder can. As the dad of a 12 year old, Lego kits today are already done for you, par the building of the actual figure/model. There is no real imagination to it now.
Back in my lego days, there were very few kits/models it was all about building something in yourself from a pile of the bricks.
Well as there will be bugger to watch on the TV tonight, I'm off to have a rummage though my sons lego collection and start building