Why is the left ventricle thicker than the right ventricle?

The right ventricle only pumps blood to the lungs, which are very near the heart. The left ventricle needs to pump blood to the eintire body, so the greater distance requires higher pressure to make sure the blood is able get all the way to its target organ and come back to the heart too. Remember some of this may need to fight gravity too (ex: blood toward your head and neck). So the left ventricle is thicker because it has more muscle, which it needs to generate more force (ie. it needs to be stronger) to circulate blood properly.