Answer:
Because of their intermediate electronegativity
Step-by-step explanation:
First of all, any element of the periodic table interacts with other atoms to complete the octet in its last atomic level and achive stability, either losing or gaining electrons.
Group 14 elements have 4 electrons in the last level, so it would seem that for they its the same to lose or gain electrons. But actually its neither, for them is easier to share the electrons with other atoms.
This is because they don't have a very high electronegativity (making them able to steal electrons) nor a very low electronegtivity (making them anle to release electrons).
This intermediate electronegativity is the responsable of the sharing of electrons.