Because while Mary was the vessel, the Incarnation was a mighty act of God, not God and Mary. And the significance of that act of God goes beyond a virgin birth and thunders through history since God made Himself man to die for our sins as the only acceptable sacrifice of atonement. God and man are reconciled, by God's hand alone, not God with man's help. That's how.
WOW.......
Wow indeed. If it wasn’t even mentioned in scripture I would suggest it was insignificant to the message of scripture.