God created the heavens and the earth, so he had authority over them.
He created humans and gave them authority over the earth. Once he had given up that authority, he could not take it back, even if humans mucked things up.
And they did. They surrendered to the tempter’s advice, so authority over the earth passed over to the spiritual powers evil.
No human could take that authority back, because sin had made them captive to the powers of evil.
God could not take the authority back for them, because he had given authority on earth to humans.
The only solution to this dilemma was for God to become a man, because only a man was entitled to authority on earth.
Jesus could have taken authority for himself and ruled the earth, but that was not enough for a loving God. He wanted to redeem all humans from the power of sin and evil. So Jesus died on the cross to set all humans free from the curse of sin.
But none of that could have happened, if God has not becomes a man in Jesus, because only a human has authority to do anything on earth. So Jesus had to be born as a baby, and grow to be a man, because only someone who has lived as we live could be truly human and therefore have authority on earth.
This is why we celebrate the birth of Jesus, not because a baby was born. Babies are born all the time. But because God became a human.
That has only happened once in human history. And had it not happened, the authority over the earth that God gave to humans would have been lost forever.