Why don’t you say "Baruch ShePtarani" for a girl at her Bat Mitzvah?

The most reasonable answer is that the Bracha is recited by the father upon completion of the Mitzvah to teach Torah to his minor son. Since the obligation of Talmud Torah is for men, the Bracha would not apply to women. If the Bracha is recited because of the obligation of educating our children to do Mitzvot (Chinuch) it is difficult to understand a distinction between bar and bat Mitzvah.