That women and men probably have different approaches to risk and reward, and that instinctively, women may choose more secure career paths over riskier ones.
There is much evidence of this, and not just in tech startups, and it's not merely anecdotal, though I think the overwhelming nature of the anecdotal is pretty convincing on its own. Vastly more men die in the line of work than women do, across many fields, for example. More men than women were pioneers by a couple orders of magnitude. More men choose the military, police or fire department as their career than women by an order of magnitude. More men become astronauts, revolutionaries, and stock brokers.
It seems highly unlikely that these disparities are entirely systemic, or based entirely on our society's norms and expectations; though I'm willing to believe that some of the differences are based on environment rather than genetics, and I hope that if I have daughters they will be given ample opportunities to choose any path they want, including startups (in whatever field startups are interesting in 25-30 years).
My point is that it is silly to suggest that just encouraging girls to take a more active interest in math and sciences will not solve this gender disparity. And, it may even be impossible to "solve" it. Women may just not want to start companies at the same rate as men. If that's the case, there's nothing we can do to "fix" it. I quoted "solve" and "fix" because it assumes that there's something wrong with people who don't want to start companies. I don't know that that's the case. We might be the ones taking unnecessary risks for things that may not be as good for long-term health, happiness, and well-being. After all, once basic needs are met, money doesn't buy happiness, and startups can be a lonely and stressful process that lasts years and may never pay off.