Wheels are stable by nature, but legs are not; a legged robot is always on the edge of falling and stays up only by correcting constantly. It senses its tilt and motion and adjusts each step and joint many times a second to keep its balance.
This active balancing is what lets legged machines cross steps, rubble and uneven ground that defeats wheels. It demands fast sensing and control, because a moment's miscalculation on a shifting surface is the difference between a recovered stumble and a fall.