You are not finished with your body when you die. Your innards will be removed from your body, and your soul (ba) will travel to your ka, which is the entity responsible for traveling to the other world. Without your body and innards preserved, however, this ka will die. This is how the Egyptians viewed death, and it is very apparent in their burial process. A peasant would surely only need a couple of garments and their sandals on the other side - a Pharaoh, on the other hand, would need much more. This is why slaves were buried with their Pharaoh, along with as many riches as they could manage to stuff in their tombs.
But why did they see it this way? Perhaps because they were unable to accept the idea that your body is disposed of and useless after death? They obviously were not receivers of divine intelligence, so we can assume they came up with this on their own. But why?