[yareal]

Mechanical advantage and impact. Weight is not just a constant force in our joints, and it's not applied evenly. As an example, imagine holding forty pounds at arms length vs wearing it in a backpack or letting it dangle to the floor. The forces on the shoulders from all three are radically different.

Additionally, when you walk or run you don't place only and exactly the force of the weight of your body down. Each foot is loaded with an impact and the forces are distributed up the leg. What your knee experiences is a dynamic and spiky load.