7 points
It’s all about maintaining 3 points of contact when moving a limb. 2 feet and a hand or 2 feet and a hip/knee when using both hands. If your feet are on the top step then you don’t have a knee or hip to keep the 3rd point of contact. And stepping on the top cap means no where to lean onto if you lose your balance (this one is self explanatory).
2 points
It’s also about the design of the ladder. The top plate is literally not a step, and the bearings for it don’t account for it to be used as a step, so the forces involved can mess up or break the connection to the rest of the ladder.
…not that they always will, just that it’s a very bad idea.
