I am not sure "Why?" is a valid question - babies with large heads in comparison with other mammals is how humans are when born; it is in our developmental genetics. "How?" might be better - which may answer why the balance between pre-birth and post-birth development shifted during evolution, which simply means those with the bigger heads at birth went on to survive and reproduce but those with smaller heads didn't. It is not just about the baby but the carrying mother; it is safe to assume those with heads too big/mothers too small did not survive. It does not even follow that those with smaller birth size failed because of their smaller birth size; the crucially significant baby born with genetics for smaller births size but capable of growing a big brain may have gotten eaten by a hyena and all human evolution after that was altered forever.
Those that were born better able to function more independently at birth and needed less parental care somehow did not go on to thrive - and not necessarily because of any direct relationship; other factors can and will be in play. We can list the advantages and disadvantages and hope we haven't overlooked some but it cannot really give a definitive answer