Comment by Gatsky

Bone is an organ with multiple functions beyond structural support. It creates a reservoir for calcium and phosphate homeostasis and of course contains bone marrow. It also has attachments for muscles and connective tissue, and adapts to environmental conditions (eg see a tennis player's forearm). A titanium bone could do none of these things. The question should really be why are bones not reinforced with metal or metal composites. In general if greater strength is needed it can be achieved with thicker bone, which has a far lower evolutionary barrier than eg metal composite bones. Additionally, the material properties of the whole biomechanical apparatus need to be considered. A super strong skinny thigh bone isn't much good if the area available for ligamentous attachment is so small that the tendons keep tearing. Most injuries involve connective tissue and muscle rather than bone.