Question

why are valves present in veins but not in arteries?

Answer 1

Veins bring back blood to heart from different parts of our body.
in order to stop the flow of blood backwards valves are present in veins.
whereas arteries are just to send blood to different parts of our body.
You may get a  doubt that blood to top parts of heart.In this case Aorta rushes blood with great force.
this allows blood to all parts of our body and we have arteries less lumen.!

Answer 2

its because ............
veins bring di-oxygenated blood from all parts to blood......the force in the veins is comparatively less in arms and legs.....to prevent backflow ,veins are there in veins................