Because there is a layer of UI/UX between the screen and "their box" that Apple traditionally hates. If there is an Apple TV with a screen (and I'm not saying there will be) then you can be sure the input switching madness, weird motion interpolation, iron-maiden remotes, and shitty setup experience will all be gone. If it ever comes to exist, it will exist to close that final gap between Apple/iTunes and your couch.