Correction. I have received responses, but they did not give me an understanding of how it could be said that Mary is not the mother of God.
God created Mary; she could not possibly be His mother. God is infinite; Mary is not. (The Bible warns us about worshiping the creature instead of the Creator.)
He used Mary as a vessel/tool to introduce His Son to earth. In that role, she was Jesus’ earthly mother.
In Luke, Mary called God her “Savior”. If she were God’s mother, she would be perfect — God-like. If so, she would not need a “Savior”.