Why do women still get brushed aside at work?

Why do you think? They're not men. From the Atlantic:

A study in 2004 conducted by Melissa Thomas-Hunt of the University of Virginia and Katherine Phillips of Columbia Business School found that women are less likely to be viewed as experts than men are, even when they have the requisite knowledge. In an experiment, 143 undergraduate business students learned about Australian bushfires and then ranked 12 items in order of importance related to surviving one. Then they were randomly assigned to groups of three to five individuals, usually a mix of women and men, to share expertise. Not only were women's opinions more often disregarded than men's, the researchers found, but women who had no particular bushfire know-how were viewed more favorably--by men and women alike--than women who knew more, because they tended to agree with the predominant view instead of challenge it. And women tended to evaluate themselves more harshly then men on equivalent performances. "It's not actual expertise," the researchers concluded, "but perceived expertise that conveys power and status."

This stuff is very hard to generalize, but perceptions can be based on all sorts of factors - industry, size of company, region of the country - and some businesses have a hard time shaking the old ways.