Why are alkaline-earth metals usually found in compounds rather than as free metals in nature?

Alkaline-earth metals are rarely found as free metals in nature because they are highly reactive reducing agents. Their chemical behavior is dominated by the ease with which their two outermost s electrons can be removed to form stable, doubly charged $M^{2+}$ ions.1

Because these elements readily lose electrons to other substances, they are almost always found in nature combined with other elements in the form of stable compounds, such as salts or oxides. The formation of these $M^{2+}$ ions is energetically favorable because the energy required to remove the electrons is more than compensated for by the binding energy gained through electrostatic interactions in an ionic lattice or through stabilization by a solvent. Due to this high reactivity and tendency to form stable ionic bonds, they do not exist in their elemental, metallic state under natural conditions.12