Because the tooth fairy takes the teeth to protect the children from a kind of contagious magic where an evil Magic user curses the teeth, then the child is cursed; but once people are dead, they’re gone and what’s left? It’s just a bag of bones; there’s nothing local on this plane of existence to curse.
Curses do not cross the black sand. That is all.
In North Ontario, the black flies want to know your next campsite location.