A better question is "why does humans only use two of their four limbs to walk?". The answer can be found by looking at our closest evolutionary cousing, the chimpanzees. They have hands just like us and use them to handle tools and climb trees. However, they also use them to walk. What makes us humans different is that we benefited so massively from standing up straight that our back feet and front feet evolved into very different limbs with unique roles. We cannot effectively use our feet as hands to grab things like a chimpanzee, and we also can't use our hands to walk.
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