Trace it back to each dropoff point.
Is the percentage of female senior software engineers appreciably smaller than the percentage of female software engineers? That's a red flag for discrimination.
Is the percentage of recently graduated female software engineers appreciably smaller than the percentage of female CS students? That's a red flag for discrimination.
Is the percentage of female CS graduates appreciably smaller than the percentage of incoming female CS freshmen? That's a red flag for discrimination.
None of it is absolute proof of discrimination, but it's enough evidence to investigate. Discrimination can take a number of forms -- maybe your high school had a CS class, and the teacher and the guidance counselor were both solid feminists, but they weren't able to enforce a non-hostile environment for women. Or maybe inertia is at play, and if there were two or three solid years with at least 30% female representation in that class it would self-perpetuate. Or maybe there are more general media and marketing forces at work telling women and girls that they cannot be nerdy and cannot be interested in computers, that they are broken and wrong without a husband and that having these skills will prevent them from getting one, that they must focus on family and not on a career, that they will end up being homemakers for half a decade or more so they shouldn't bother getting a high-skill job or worry about education overmuch.
Just saying "people are different -- who cares?" hides these problems.