Probably the most common Catholic title for Mary, mother of Jesus, is "The Blessed Virgin" or "The Blessed Virgin Mary". But "Blessed", in Catholic hagiology, is typically used to denote people who are not saints yet:
Blessed is the title for someone who has been beatified. ... an earlier step in the process toward sainthood.
(US Conference of Catholic Bishops, How To Cover The Catholic Church)
Catholics consider Mary the holiest of all human beings:
By her complete adherence to the Father’s will, to his Son’s redemptive work, and to every prompting of the Holy Spirit, the Virgin Mary is the Church’s model of faith and charity. Thus she is a "preeminent and... wholly unique member of the Church"; indeed, she is the "exemplary realization" (typus) of the Church.
(Catechism of the Catholic Church, paragraph 967, quoting the dogmatic constitution Lumen Gentium, paragraphs 53 and 63)
This being the case, why is it that Mary is not—or at least not usually—given the (apparently highly appropriate) title "St. Mary"?