Why does not the father of a girl who has reached the age of her Bat Mitzva make a bracha as he would have done if it was a boy?
The Rema (OC 225; 2) states: “there is an opinion to say blessing of Baruch Shepetarani when one’s son becomes Bar Mitzvah and better to be said without Hashem’s name” This Brachah does not appear not in the Talmud nor the Rambam, Tur, Shulchan Aruch. The earliest it appears in literature is by the R Yehuda ben Baruch, a contemporary of Rashi. As for saying this blessing for a girl; from the words of Rema it appears this applies to boys only, the Pri Megadim (ibid 5) questions the custom to say only for boys as there should be no difference between boys and girls, he eventually finds a difference, it has to do with the fact that the level of educational responsibility (Chinuch) is on a lower level than boys. Though his arguments can be challenged, the common Ashkenazi custom is to say for boys only. The Sephardi Halachic authorities hold by the opinion that since the obligation and responsibility is toward the girls as well, the Brachah can be recited for girls especially since it is recites without a Brachah (Yabia Omer 6; 29). For more you should see a perfect summary of sources on this Brachah here http://www.daat.ac.il/daat/kitveyet/sinay/baruh-2.htm