I think that potential pregnancy is a factor. The hours traditionally worked by startup founders could be detrimental to health, and the inability to take prolonged breaks from work at CEO level is probably a deal-breaker.
(Some of the best salespeople I know are female, particularly in consultative selling.)
RE-EDIT: Sarcastic thanks for the downvotes. And sincere thanks to whoever upvoted. I feel better!
It's claimed (by?) it takes an average of 7 years to exit your startup. That is a large chunk of time for women with a ticking biological clock. I do realise of course that it is possible to have kids in to your 40s, but most women choose not to do so.
Honestly, I see my "grad school clock" as an equally big, if not bigger, problem than anyone's biological clock. When you do a startup, you have to choose your priorities -- the implication is that neither kids nor grad school are amongst them.
(1) The average is 6 years, but that includes lots of lazy people, future college teachers, people who enjoy grad school, etc. Potential startup founders would almost certainly be faster.