Why are elements of group 0 chemically inert?

Elements of group 0 are called noble gases. These elements have stable arrangements of electrons in their outermost shell. The chemical reactivity of a group highly depends on the number of electrons in the outermost shell. A stable element generally has 8 electrons in their outermost shell with some exceptions of elements with higher atomic masses and helium. If the element has less than 8 and greater than 4 electrons in their outermost shell of the valence shell it will undergo a chemical reaction to gain some electrons from a different element whereas if it has less than four electrons in the valence shell it tends to lose them all in order to gain a stable state. 

However, in case of noble gases or group 0 elements the outermost shell contains 8 electrons (2 in case of Helium) which is a stable state configuration. Hence, they are chemically unreactive or inert.