Why are metamorphic rocks formed by contact metamorphism usually not dense or resistant?

Metamorphic rocks formed by contact metamorphism are usually not dense or resistant because they are formed at relatively low pressures and temperatures compared to regional metamorphism. The short duration of the heating process in contact metamorphism does not allow for the recrystallization and reorganization of mineral structures that contribute to denser and more resistant rocks. Additionally, the presence of fluids and gases in contact metamorphism can facilitate alteration and weakening of the original rock material.