Never mind the feet, why are skeletons still holding together? That is the better question. Realistically, it's next to impossible for this to occur. Bones are held together by ligaments, cartilage, muscles, etc. When all of this decays there is literally nothing left to hold them to their ergonomic configuration. So, a cranium (or skull) should not still sit atop the cervical vertebrae (the neck). Likewise, the arm (or humerus)/legs (femur) should not remain in their respective sockets. In the last mission of the Cosmodrome, for example, there is a skeleton sitting on a chair by some computer monitors. The entire skeleton is almost intact and fully together. This is not possible, it should be a big pile of bones.