Cause they'd rather people be like that Sears Commercial where you see the guy on the couch watching TV, and the wife walks up behind him and casually asks "What game are you watching?" The guy replys, "All of them", and the camera view changes and you see about 20 completely different TV's stacked up like a retail store.
Some HDTV tuners do have a screen capture guide where you see 16 still shots from different stations at the same time.
In reality, manufacturers probably realize most people cannot figure out how to use their PIP, and 4 channels would wind up being a support nightmare.