Among other reasons, there are many aircraft without seatback IFE systems and many airlines that have a mix of aircraft with and without them. There's also a move away from them since they add a lot of mass and use power, and nearly every passenger now is carrying at least one wifi capable screen that's probably higher quality than the IFE, particularly if the seatback IFE is more than a year or so old. For an airline to do it your proposed way in seatback IFE equipped aircraft and old-style on non-seatback equipped aircraft would add to complications with service consistency and they'd probably get a lot more complaints.

On AS you can pre-order some of the meal items and pay for them on the aircraft (and change your mind - you can pre-order and then not buy the item and they'll just sell it to someone else), but they still go around with the cart.