The actual assumption is to avoid supernatural explanations along with the infinite number of other explanations that don't have any evidence for them, for the time up until some evidence is shown.
This is simple logic. This is the same reason when your newspaper is missing you don't assume that a ghost did it before you assume your neighbor stole it. If some sort of evidence emerged for the ghost, you'd be all over it (as would a scientist).
The paper man's first defense is the dog did it.