Probably because after you do that, and hook up a monitor and a printer and an external harddrive, you have basically what's referrered to in todays parlance as a "laptop."

The Touch (as a mobile computing platform) is for people who don't want this "laptop" I speak of. Hell, following your logic further, you could permanently afix the laptop to a specific location to no longer worry about moving it. We have those as well, they're called, "Desktops."