why does the superior vena cava, inferior vena cava and the pulmonary veins do not have any valves? valves help in proper blood flow, but these are alos main vessels but why dont they have it. even though IVC have a valve its not functional.. why?

The superior and inferior vena cava and the pulmonary arteries have no valves because no back flow of blood is there in them as the blood is flowing from them into the atrium. Gravity does not cause any back flow and the sucking action of the atrium pulls the blood into it causing the valves if present to be rudimentary.