I don't see that it is a boys-only club mentality. I don't see why that would come into it. Of course if that is the case I would argue that something ought to change.
My point is that generally by the time you reach that level in a company you're probably in your 40's, have worked hard and made sacrifices (such as not spending much time with your family while you work). It is simply the case that if you have decided to take time out of work to raise a family that you are going to be left behind so far as that goes. Our society doesn't compel that, for the most part people do it voluntarily. Again it's important to state that I don't think women should raise a family if they don't want to, or that men shouldn't take that role, it's just generally what happens.
By the time you have worked somewhere for long enough to be eligible for the boardroom, there will probably be less eligible women for the role because a lot of them will have taken time out or cut their hours to start a family, while the men continue to work. Not all of them do of course, but it will make a difference to the overall statistic.
43% of women take time out of their career to raise children if this is the case you have at least part of your answer as to why more men make it into the boardroom. Not because they don't want women in there, but because by that point many women have dropped out of the race.
A lot of executives work 72 hour weeks in order to get to that point you have to sacrifice time with your family, or not have one. I'm not saying women are incapable of putting in that kind of work, rather that on the whole they make the opposite sacrifice; family over career. There's no compulsion in that, it's just the preferences people tend to have. It don't think it is indicative of systemic bias.