It sounds like you were on a fairly old United plane (CRTs in the aisle). While there are newer planes with seat-back screens, their newest planes eschew any sort of seat-back entertainment completely. At least on domestic flights, it's been a while since I've flown internationally.

This is a deliberate decision that airlines are starting to make. If most passengers are bringing a wifi-capable device (ipad, phone, etc), then why go to the extra expense (and weight) to support seat-back screens?

Now, my main experience with this has been flying within the US where none of the newest planes have screens. Internationally, there may still be screens included in newer planes, but when screens = weight, and weight = $$$, it's not surprising that they are getting removed. Moreover, screens = depth, which when removed can make room for more rows, which means more money.

If this electronics ban takes hold though, I'd expect for there to be a big rethink about this.