My suspicion is that it has something to do with why Goujun is about two feet long.
Such as, heavenly beings are actually a different order of being, and although they manifest in human or humanoid shapes, their composition is actually quite different. This could also explain why they're not allowed to kill--it might be that there are real and not just legal repercussions for that. Which could in turn go a long way towards explaining Tenpou--who, as WildeLamassu just reminded me, is a real anomaly among kami for his lack of innocence.
So if their composition is different, it might be of a nature that has faulty compatability with the human and youkai world. Maybe they have to do something to themselves, a transformation or confinement which limits both their form and their ability to interact with that world; to use their full abilities as they otherwise would be able to, but also protects them from some effect of the world, or combined effect of the world on their power, on their selves.
Beings such as the Boddhisatvas and the itan might be an exception, having some mortality in their natures to begin with.
Did I mention I have no idea what I'm talking about? Just spinning speculation here. ^.^
Email: Nightfall@suntemple.org