Here is the answer:
Human body is covered with short and long hair but we can see only some of them easily. There are oil glands at the bottom of them. Oil coming out from these glands called sebum creates a waterproof layer.
Keratin layer is at work here. That’s why our body is waterproof. Our skin softens with it. There are no hair on our finger tips and soles because they are used constantly and hair formation is hindered.
At our hairless body parts, there is no keratin layer. These areas have thick skin layers.
When our finger tips and foot soles are in water for a long time, water penetrates under the skin.
Water seeks for a place, yet the thick skins on finger tips and soles don’t not allow water pass. After a while wrinkles are formed.
Actually there is a keratin layer in our hands that prevents wrinkling. However, it disappears as we wash them very frequently. As a result, our skin lets the water in, and the skin containing water wrinkles.
As we see, it only happens at our hands and soles because most other parts have hair and oil glands.