I don't think it's simple enough to give a single answer to or to be able to answer entirely (and I'm not an expert at all), but I think there's a lot of factor

I don't think it's simple enough to give a single answer to or to be able to answer entirely (and I'm not an expert at all), but I think there's a lot of factors...sexism, the competition at the top level and the natural aggressiveness of men, rewarding aggression in business, the negative view of babies and women with families in culture (again, sexism). Honestly though...I think one of the biggest factors is that success in school does not predict success in a job. The traits rewarded in school do not correlate to the traits rewarded in high-competition jobs. Being quiet, listening, doing your work on time, not challenging the status quo, parroting things that teachers want you to, not challenging others...none of those things are traits you think of when you think of CEOs. Not that I think that aggressiveness necessarily SHOULD be what we're rewarding as a culture, but when the only goal is profits...it's hard to think that those things make a top company. I'll probably get down-voted for mentioning him (and that's fine, I don't care), but Jordan Peterson has some really fascinating talks about personality traits between men and women and why personality traits in men lend toward more aggressiveness which is rewarded in competitive business.