Why are metals always monoatomic?

It is a convention. The metallic elements have what is known as a metallic bond that consists of the atoms are united by a current or "sea" of electrons which are free to circulate in all the "piece" of metal.

In strict sense, there is not such thing as a molecule of sodium or iron, but all the material is bound by that sea of electrons that was mentioned. Thus, for the purposes of chemical reactions the metal element is counted as an individual atom, i. e. monatomic.

The type of bonding in most metals is metallic bond. the atoms are held together by powerful electro static force in a sea of electrons in that respect they may be regarded as a giant molecule but in chemical reaction they are considered to be monatomic. there are some exceptions, mercury where you convalent bonding Hg2.