“Who cares whether the title is in Scripture or not?”
Only those who treasure the words of God as truth.
Only those who base theology on Scripture.
“The important thing is that the title has a rational justification, based directly on what Scripture tells us about Jesus and Mary.”
Well, no Arthur.
Why did God leave that title out of Scripture Arthur?
Ask Him.
What I do know is that the title expresses a truth that is in the New Testament: Jesus is God and Mary is His mother, and she intercedes with Him on our behalf. For example: The marriage feast at Cana.