"For whatever reason" is due to safety! You aren't supposed to watch movies while driving, and the video would be distracting. Nissan SUV's with DVD systems only play the video on the rear headrest screens for this reason, even though the DVD disc is put into the main radio unit up front.
It is meant as a parking aid only, not to make sure you are centered in your lane at 65mph. If that's a must have, get a new Leaf with ProPilot assist. You can't watch the cameras on the video screen but it will keep you centered in your lane.