Being able to take 1+ years off when you have children without serious career damage is more important to woman than men.
The research I have read suggests that women are much less likely to leave a job they enjoy and where they have made friends in order to get a pay rise than men are. It's not much of a leap for women to consider this issue before entering a field.
Likewise there is considerable research on women being more risk averse than men and surely this at least has an impact on joining the startup world.
My last two paragraphs basically boil down to a question for you: why are women who are, in your opinion, not becoming software developers because of sexism still entering into roles that interact heavily with these sexist software developers?
That doesn't mean that the solution to the problem is in university enrollment, but it does explain why it's easier to find women in non-tech roles in software companies: it's because those roles don't require an early life commitment to a career in technology.