The Bag Jump is set up according to the regulations of the company who we've purchased it from. If you were to look at other Bag Jumps, you would notice that all of them are set up very similar if not exactly to how ours is. It's a safety/liability thing. I understand your thinking though, but I think a fall onto an airbag from higher up would risk a greater injury, and the whole point of the bag is to offer a safe way to practice arials before trying them on the snow, to lessen the chance of an injury.
Hope this info helps you understand it better!