I had always assumed it was a morphic field thing. Some FROM DEEP WITHIN THE RED OR GREEN type shit where the ideal conduit for willpower and life is this bipedal structure.
Life naturally takes on a similar shape as the most optimized use of its capabilities and intellect. Evolution settling into a universal cosmic mold.
We don't look like Kryptonians, Kryptonians don't look like humans. We're all just walking upright because it's the best place for most of our brains to be.