As opposed to what, diatomic? Some elements, which are usually gases, form diatomic molecules because the combination is lower in energy than existing as single gaseous atoms. We can pretty much say that elements react the way they do because the products will be at a lower energy than the reactants. The same is true for the formation of diatomic molecules. 

Metals form extended "moleucules", a vast network of atoms, (like diamond, for instance) in which each metal atom is bonded to one or more adjacent metal atoms. From that standpoint, metals are NOT monatomic.