Why are veins thinner than arteries?
Veins have much thinner walls than arteries, largely because the pressure in veins is so much lower. Veins, like arteries, have three layers. Despite all these layers, you can find less smooth muscle and connective tissue here. The walls of your vein walls are way thinner than artery walls because the pressure of blood returning to the heart is relatively low.

Biology of the Blood Vessels – Biology of the Blood Vessels – MSD Manual Consumer Version

https://www.msdmanuals.com/home/heart-and-blood-vessel-disorders/biology-of-the-heart-and-blood-vessels/biology-of-the-blood-vessels#:~:text=Blood%20flows%20from%20the%20capillaries,of%20fluid%20in%20them%20increases.

Biology of the Blood Vessels and Heart and Blood Vessel Disorders – Learn about from the MSD Manuals – Medical Consumer Version.