They've answered this before. It's because of the leg models and walk/run animations. The calf muscle flexes a certain way based on the angle between leg and foot. (Which is typically why heels make a person's legs look better). Removing the sandals would present a problem because the legs would then look off. Or they'd clip into the local fauna that's been designed with the idea that our characters will always be wearing some kind of footwear. You can already see some of it happening if you zoom in by stairs and other little hills.