Because old gens were big leaps in terms of technology.
2D->some 3D->full scale 3D, advanced lighting->shaders, motion capture, full voice acting, high res models and textures that are downsampled for maintaining framerate.
But new gen will be about using those advanced assets in their full potential and better lighting, interaction etc. It wont increase cost exponentially, because there is no planned feature to do so [like full voice acting or million poly models].
They are already making games like CG movies, with almost the same technology.
Guys think first, comment later, because we will just repeat the same argument over and over in every page.