Why are embalmed bodies hard?

Many of them are dehydrated (ie: ancient mummies). In others, the fluids have congealed, making them feel much firmer than living or recently dead flesh.

Dead wrong! (No pun intended) Embalmed bodies feel firm because aldehydes (formaldehyde, glutaraldehyde, etc.) preserve the body by linking together the protein chains present in the flesh. These "link sites" also happen to be the same sites that are broken down by the various bacteria that contribute to putrifation and decay. That's why embalming retards decomposition.