Why do soccer players always wear another pair of tight white bodybuilding pants under their jockstrap?

Not always white, those are scooter pants. These pants are designed to reduce energy loss in soccer by reducing the shaking of the thigh muscles during running. They also protect the player from bruising the sides of the thighs or buttocks when shoveling the ball. Players will shovel pants worn inside the sports shorts, tight and slightly longer than the outside of the shorts, almost extending to the knee (FIFA regulations, shovel pants color and jersey pants color).