This question made me crack up. And to be honest, I've never thought of it that way before.
Oh wait, it just hit me....because the numbers "9" and "6" don't fit into each other like "6" and "9" do. Get it?
If I'm doing a "96" with someone we'd be laying back to back with not much happening except maybe some head to butt rubbing action.
Damn, I'm brilliant ain't I? And it would appear that I have given WAY too much thought to this question. Ha.