You could also ask the same question of boys. In fact back in the 30s and 40s this question was asked and most of the boys did not continue in yeshiva past high school. Historically we can see the result of this. Most of these boys did not remain frum. Today of course it’s a given that a boy remain in yeshiva post-high school. The result is that most boys remain frum. Learning Torah as long as you can will strengthen a person’s base of yiddishkeit needed to fight the yetzer hora of the outside world. An additional year at least of kollel adds the foundation of a Torah-true life to the couple.
Girls’ seminary accomplishes the same thing. This girl will be a mother to yiddishe kinder. She is the one who is home with them as they grow up, before they start school. The mother teaches them krias shema and brochos. The mother and wife needs a firm basis of Torah and yiddihkeit. The holy mother of the Bais Yakov system of learning for girls, Sara Schenirer zechusa yogen oleinu, realized that in a modern world a stronger basis for yiddishkeit for girls was required. Historically we can see that this was true. Seminary is the capstone for a girl’s Torah education. And it also becomes the foundation for her Torah life after she marries, respect for Torah and mitzvos. All three of my daughters as well as my wife attended seminary. True they all aspired to become teachers, which they did. But it also gave them the strong foundation in yiddishkeit which I spoke about.