Why are the inert gases termed #"zerovalent"#?
Because generally they do not form chemical compounds.
Traditionally, valence was defined as the number of hydrogen atoms that could bond to a particular element. Inert gases tend to form compounds with only the most reactive and oxidizing elements, namely oxygen and fluorine.
The electronic structure of the inert gases, which feature a full valence shell of electrons, can rationalize their poor reactivity.