It's not completely brain development, look up the stepping reflex in human babies. Humans are just as neurally pre-wired to walk as foals are on day one but we

It's not completely brain development, look up the stepping reflex in human babies. Humans are just as neurally pre-wired to walk as foals are on day one but we're also born long before we're anywhere near strong enough to do it, it takes at least another 6 months of physical growth and strengthening out of the womb before babies even try.