It’s all about blood circulation. If you’re standing still, blood will accumulate in your legs/feet, because your leg muscles aren’t using much and thus gravity takes over and the pressure causes pain
Whereas walking keeps the blood circulating because your leg muscles need fresh blood in order to move (they need oxygen rich blood) and thus allow the pressure from gravity be relieved.
Eli5: not moving? Valves shut -> pressure buildup -> pain
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