Why do groups among the main-group elements display similar chemical behavior?
It is due to the number of valence electrons.
The atoms of elements in the same group have the same number of valence electrons. It is the valence electrons that are involved in chemical reactions. Atoms can gain or lose valence electrons to form ions and ionic compounds, or they can share valence electrons to form covalent substances, such as diatomic molecules or compounds. Atoms of elements in a group largely chemically behave the same way due to having the same number of valence electrons.