[Perdition]

>the problem seems to be that their muscles cannot support weight or balance.

It does? Newborns can support their own weight when hanging. Most toddlers can support their own weight standing long before they develop the coordination required to walk. Babies lack fine motor control.

Our ancestors, with less complex brains, would have been able to walk shortly after birth, hence the walking reflex.

I suspect the issue is that the complexity of modern human brains delays the formation of the neural circuits required for walking because a whole bunch of complex features are developing at the same time.

[Stwerp]

I do see your point but still, a newborn cannot support their head by themselves. Having the muscle ability to be able to hold ones head up is a fairly good prerequisite for being able to walk.