Carbon and silicon are tetravalent but ge sn pb show di valency. Why

Carbon and silicon both are tetravalent elements as compared to germanium, tin, and lead which are divalent.

That's because Ge, tin, and Pb show inert pair effect and has a greater nuclear effective charge on the 's' electrons due to poor shielding effect ... That's why these elements are not able to share their valence electrons while carbon and silicon does and show "catenation" which is the ability to form long chain molecules.