Running is the act of a panicked person. Everyone sees everyone else running theyll think they're in danger. Theyll start cashing out their stocks and withdrawing their live savings so the banks and the stock markets collapse and finally realize we can live without the curse of consumerism and execute all advertisers and move forward into a new golden age of humanity.
So in summary, we walk because THE MAN has our legs in chains
Humans are endurance predators, not speed predators. We are biologically tailored to move at a slow, but energy-efficient pace so that we can aggressively stalk our prey until they tire out from running. That's how Neolithic men hunted.