By wikipedia "a mummy is a deceased human or an animal whose skin and organs have been preserved by either intentional or accidental exposure to chemicals, extreme cold, very low humidity, or lack of air, so that the recovered body does not decay further if kept in cool and dry conditions. Some authorities restrict the use of the term to bodies deliberately embalmed with chemicals, but the use of the word to cover accidentally desiccated bodies goes back to at least 1615 AD (See the section Etymology and meaning)."
I'm not an expert but my explination would simply that a soul of a deceased human has it's connections to the earth body.
Hope that helped
