Why do evangelicals or protestants deny the idea of Mary as Mother of God?

I mean Mother of God son.. if Jesus is truly God and Human.. I will say Mary is Mother of God.. Elizabeth called to Mary in the first chapter the Mother of My Lord..was she referring to Jesus as God? because Saint Paul said Jesus is the author of the creation.. then Mary was Mother of God because she gave birth to Jesus not just his humanity also his divinity.. Jesus was born as divine and human.. if Mary was mother of the humanity of Jesus then when Jesus got his divinity?