Because they have complete electronic configuration
Explanation:
Group 18 elements are known as the noble or inert gases. The elements in this group are non-reactive because they have complete electronic shell configuration.
Electronegativity is the tendency of an atom to attract electrons.
The reason why atoms attracts electrons is to have a stable configuration like those of the noble gases.
Since the noble gases do not require any extra-electrons to attain stability, they have 0 electronegative values.
