I have a problem with the premise that most managers fail or are poor. Management is often judged by those managed based upon how the managers does by them. From that perspective yes, most managers suck.
Much of a managers job is balancing the needs of those above him with the needs of his people... balance is probably the wrong word... carry out the orders of those over him while minimizing the anger and resentment of his employees. Often those given the title of "good managers" just have less stress from on high.
But there are a lot of terrible managers out there. But for every one of them there are a half dozen employees who complain incessantly about things beyond their bosses control.
As for women, I think there is a bit more to it than this. As responsibility levels rise, the term "maternity leave" becomes more and more problematic. I realize it isn't fair or reasonable. But the reality is that if you have a high level of responsibility, taking long term time off becomes a huge problem.
I suspect this also has a lot to do with the pay level differences. When a large percentage of women take off anywhere from months to often years to have children, it's hard to expect their pay to be equal. I am all for equal pay for equal work. But equal pay, regardless of the fact that I stayed on the job for 10 years while she took 3 maternity leaves.... seems more than a bit unfair too.