Why does argon exist in nature as individual argon atoms and chlorine exist in nature as 2 chlorine atoms covalently bonded together?

Argon has 8 valence electrons and no extras, it does not require a bond in order to fill its shells, its satisfied by itself.

Chlorine is missing 1 Electron, if it connects with another Chlorine it will be satisfying both of their needs with a Covalent bond.