Because our bones are made of calcium, why don't our bones react to water?
Calcium does react with water, but our bones aren't made of calcium metal. Bones are a collagen structure filled in with mostly calcium carbonate and some calcium phosphate. Calcium carbonate and calcium phosphate are ionic compounds that have different properties than neutral calcium. Neither is very reactive or soluble in water so bones don't dissolve in their aqueous environment.
Some calcium carbonate can be leached out of the bones because of an imbalance in the blood's bicarbonate buffering system, leading to loss of bone density. This isn't caused by the calcium compounds dissolving in water. It's a shift in equilibrium in response to a change in the blood chemistry.
Elements that are reactive are more likely to be found in compounds than in their unreacted form.