[miahi]

You have to prepare for it - have a full battery (most of the planes don't have USB or the plug doesn't supply enough current), have the movies/music already on the phone/tablet. Not everyone is using the phone for this kind of entertainment, so they have to prepare before, and because most of the entertainment is now "in the cloud" and consumed on the fly, you have to know how to copy them for offline use.

Also, the in-flight entertainment system is better positioned than a normal phone or tablet - you need a special case or something to hold a tablet/phone in a kind of vertical position on the little table (also needs preparing for), and you have to look down to it all the time - which is bad for your back, especially in the crammed plane seats.