Why are the walls of arteries proportionately thicker then veins?

In general, the walls of arteries are thicker than those of veins. The tunic media in particular tends to be much heavier and contains substantially more smooth muscle and elastic tissue. This anatomical difference reflects a functional diffeerence in the 2 types of vessels. Arteries, which are closer to the pumping action of the heart, must be able to expand as an increased volume of blood flows off into the circulation during diastole. Their walls much be sufficiently stong and resilient to withstand such pressure fluctuations.

The tunica media is thicker in the arteries mainly for maintaing blood pressure and continuous blood circulation.