i've always wondered that too perhaps when we come accross a dead animal it has usually been killed rather than died of natural causes and the usual carrion eaters don't fancy eating something that has died in an unnatural way in case they get diseased or something.Or maybe the animal has a feeling that it is going to die and 'crawls away and dies' the same feeling we get if we've had a skinful !! It does remind you of that old question Where do elephants go when they die?