Why Destiny doesn’t allow cross-play

In which some corporate jackass attempts to befuddle the issue as to why I can’t play Destiny with my son unless I buy another PS4 for the house. I think they are losing a bunch of repeat business with this policy for no real reason other than some vague sense of platform lock-in. I would absolutely already own a second Destiny license for PS3 if the no-cross-play policy were not in place.