bio fact.

the skin on our hands and feet is anchored to the underlying tissue differently than the rest of our skin, making it easier for us to grab things and walk. these anchors can't be seen when your skin is in its normal state. however, when you stick your hands and feet into a hypotonic solution, like most bathwater and dishwater or swimming pools, your hands begin to swell everywhere except the points of anchorage. this causes your hands to wrinkle.

that is why your hands and feet wrinkle upon being in the water for a long time, but the rest of your body doesn't... lack of anchorage points.