I think that a big part of the problem is that the workspace is run in a very macho way. There's still a lot of chest thumping and "my dicks bigger than your dick" kind of mentality in a lot of senior level meetings and it is deeply unpleasant to work in such a way - it certainly goes against my work ethic.
I also found that I had to shout louder and longer (metaphorically) to be heard and that when a female adopts a confrontational attitude (e.g. As a nonexec director, when i confronted a non performing management team and tried to call them on it) there is a lot of eye-rolling and a real unwillingness to take such points on board yet when a male non-exec in same position starts to back you up, all of a sudden management are listening.
I found that when I took a maternity leave, as well as deciding to stay at home for a bit because of the difficulty and expense of childcare, I also had no compulsion to make a hasty return to such an environment.
Another issue, I wonder, is the attitude of women in senior positions once they do break through that glass ceiling. Now my experience is only based on one female manager from hell, so statistically invalid. However she made it far more difficult for women to progress than men. I have heard of similar cases from other women who had female bosses so I do wonder that if these senior female managers, because they have had to be so aggressive in order to get to the top, then ironically make it more difficult for other women to do the same (please note I am not trying to say that all women are like this!!
Sorry, I've written a long and rambling response, but just wanted to relate some of my experience to explain why it might be more than childcare issues which are at the root of this phenomenon. I should also add that I have worked some fabulous people, male and female, so am not trying to make too much of a sweeping generalisation!