Because the normal and natural connotation of mother denotes ontological oneness, making Mary as a goddess begetting a God after her nature.
While Mary gave birth to and mothered the incarnate Christ, she contributed nothing to His Divine nature, and owes her own existence to Christ. Giving her the formal title Mother of God disrespects how the Holy Spirit describes and gives titles to created beings, which works against exalting them as RCs extremely do with Mary, thinking of her "above that which is written," which the Spirit warns against. (1Co. 4:6)
The terms "God-bearer" which some of the ancients used is less misleading, but Caths largley refused to use that in deference to the term "Mother of God," as they seek to exalt her, ascribing things to her which are nowhere ascribed to any created beings, but many parallel the those of Chris t.
What Ratzinger wrote about the title Co-redemptrix” departing to too great an extent from the language of Scripture and of the Fathers applies to is akin to "Mother of God " departing to too great an extent from the language of Scripture
When asked in an interview in 2000 whether the Church would go along with the desire to solemnly define Mary as Co-redemptrix, then-Cardinal Ratzinger responded that “the response of the Congregation for the Doctrine of the Faith, is, broadly, that what is signified by this is already better expressed in other titles of Mary, while the formula “Co-redemptrix” departs to too great an extent from the language of Scripture and of the Fathers and therefore gives rise to misunderstandings” (53).
He went on to say that, “Everything comes from Him [Christ], as their Latter to the Ephesians and the Letter to the Colossians, in particular, tell us; Mary, too, is everything she is through Him. The word “Co-redemptrix” would obscure this origin. A correct intention being expressed in the wrong way. “For matters of faith, continuity of terminology with the language of Scripture and that of the Fathers is itself an essential element; it is improper simply to manipulate language” (God and the world: believing and living in our time, by Pope Benedict XVI, Peter Seewald, Ignatius Press, San Francisco, 2000, p. 306
While Mary gave birth to and mothered the incarnate Christ, she contributed nothing to His Divine nature, and owes her own existence to Christ. Giving her the formal title Mother of God disrespects how the Holy Spirit describes and gives titles to created beings, which works against exalting them as RCs extremely do with Mary, thinking of her "above that which is written," which the Spirit warns against. (1Co. 4:6)
How did you manage to say that so well?
“Because the normal and natural connotation of mother denotes ontological oneness, making Mary as a goddess begetting a God after her nature.”
But Jesus has both human and Divine natures. Mary gave birth to a being that is ontologically fully human. God the Father provide the nature that is ontologically fully Divine. Jesus is both/and, not either/or. Mary gave birth to a person who has has both natures, but she did not provide any of the Divine nature, so it is not required that she be a goddess in order to still be Mother of God. Remember, this is a singular event in history, so the normal connotations go out the window.
Aquinas probably has the definitive answer on this, but I won’t have time to search the Summa Theologica for a while.