becasue most people that buy a sony tv with built in android TV arent too worried about the storage space... the majority of them just use a few apps, and will usually have space for more apps. games are an afterthought to most of these users, and the ones who want to game will get the shield, or already have consoles/pcs.
Its really just easy access to netflix/hulu/etc....
Makes them cheaper I'm guessing. You can always expand the storage with a USB or microsd.