The more outturned your feet end up, the less your feet/toe/calf muscles have to work (try it). It also alters your posture a bit and changes the stability muscles needed to stay standing up. This reduction in muscle exertion is most likely the reason you see so many out of shape people with duck feet. As my gait finally started to change from switching to minimalist shoes I noticed that when I walked less duck-feeted that I did have to engage more muscles (especially to retain my normally fast gait) but this change also drastically reduced the amount of shock I would take from walking around.
"You can demonstrate the purpose and limits of human digestion with a simple experiment: eat a steak with some whole corn kernels, and see what comes out the other end. It won’t be the steak."