The first is because some thigh high socks shrink very evenly after washing. They are woven from great cotton yarn. These cotton yarns have no impurities, and there is almost no deformation of the socks even after shrinking, . Similar to gold, 24K gold is very soft, but the hardness of 18K gold become much higher, which causes their ductility and color to be very different.
Another reason is because the elastic cord in the socks can resist the corrosion of laundry detergent. They are usually made by high quality nylon stretch threads. They will not be affected by laundry detergent. They can still maintain very good elasticity after multiple washings. They will shrink like new stretch threads.
So if you don't plan to wear disposable thigh high socks, then your thigh socks should be woven from underwear-grade cotton yarn. They are soft and shiny, and have good elasticity and easy to restore.