"Magic has its quirks, both godly and--" Wow, is this a sore topic "-- both sorts," Oliver amends. To cover up for his slip, he continues talking. "It doesn't aid a fox-skin's survival that their magic turns them mad, yet they are. Nor were wolf-skins helped by becoming ravaging beasts every full moon."

Demons can't function perfectly in a world not made for them, is one of the explanations.