This isn't "girls being sexist." Like other folks have explained[0][1][2][3][4], women often feel uncomfortable in spaces where they're the minority and might even have negative experiences in situations like that. And thus, they are put off by joining those groups in general. Groups that encourage women to join help women feel less intimidated by the idea of joining those groups.

[0] https://news.ycombinator.com/item?id=30277540 [1] https://news.ycombinator.com/item?id=30278113 [2] https://news.ycombinator.com/item?id=30276633 [3] https://news.ycombinator.com/item?id=30299883 [4] https://news.ycombinator.com/item?id=30276633