After reading the question, I tried it on myself. I move my thumb and little finger separately from the rest of my fingers without any problems. Perhaps the fact is that in kindergarten we often did developmental exercises with our feet (for example, we wrote our name on a piece of paper in pencil). Or maybe it's a genetically transmitted skill, like rolling the tongue into a tube.
Lean evolution. If a function is not needed, it is not. People walk with their feet, they don't cling to branches, they don't put apples in their mouths. So finger independence is not required. So the anatomy is adjusted.