Same for rivers, they are carved out just where the water needs to go.
It is not the material, but the laws in our particular universe that are so shaping. Rivers move fastest where there is a restriction (unlike traffic, that stalls). The faster flow causes abrasion, and over time evens the flow. The same law, seen in the Venturi effect is also is one of the 4 reasons, why wings can lift us into the sky.
Love the laws that squeezed through the big bang.