The heaven, even the heavens, are the Lord's: but the earth hath he given to the children of men.
"But when the fulness of the time was come, God sent forth his Son." If you study that out, you'll find that it means at the appropriate moment. Jesus could not have come onto the scene until then; all the prophetic utterances hadn't yet been spoken. For example, it had to be prophesied that Jesus would be born of a virgin. That was critical because Jesus would not be just a good man. He was God manifest in the flesh. He was a God-man.
Then said Mary unto the angel, How shall this be, seeing I know not a man?  And the angel answered and said unto her, The Holy Ghost shall come upon thee, and the power of the Highest shall overshadow thee: therefore also that holy thing which shall be born of thee shall be called the Son of God.