The crack starts from below, not the surface
Heel skin does not thin and split because it is dry. It splits because it is compressed repeatedly from below and sheared laterally with every step. The fat pad under your heel, a dense, shock-absorbing cushion, spreads outward when your body weight lands on it. Skin at the rim of the heel has to accommodate that spread. When it cannot, because it has thickened into rigid callus, it cracks instead of flexing.
