Why don't we find animal corpses and skeletons all throughout the woods?
Large predators can break up the bones to get at the marrow. This disperses the skeletons. But for the most part, bones are a good source of calcium, and are eaten by rodents. You can see evidence of this when you come across the gnaw marks that rodents make on the plastic boards made from recycled milk cartons. As for the reason that human skeletons are particularly resistant to this destruction, I have only a guess. Perhaps the first stage of destruction, the dispersal by large predators, is less common because these predators have been trained to avoid humans. I would also guess that many murder victims are concealed by the murderer.