Essentially the same reasons that, for a long time, women were underrepresented in most of those fields. Have you read about Sandra Day O'Connor's experiences as a newly-minted lawyer. Despite solid credentials, she had a helluva time getting a job. From her Wikipedia article:

After graduation from law school, at least forty law firms refused to interview her for a position as an attorney because she was a woman.[17] She eventually found employment as a deputy county attorney in San Mateo, California, after she offered to work for no salary and without an office, sharing space with a secretary.[17]

On December 20, 1952, six months after graduating from law school. Let's see. That happened 64 years ago. I would say that things are quite different for women these days, so your example is no longer very relevant.

Talk to some black people about it. (Yes, I have. I was fortunate enough to be raised by parents who made living in a diverse community a priority and to go to a magnet HS with girls of all shades from all over the city of Philadelphia.) They'll tell you about the barriers they've encountered.

Well, as you are so fond of pointing out: the plural of anecdote is not data. Yes. And I am Jewish so I could tell you about some of the barriers that I have encountered. I am also short and that has caused me to encounter barriers. The point is that everyone is prejudiced about something. The Civil War ended 152 years ago. The civil rights movement happened 49 years ago. We have had 49 years of affirmative action and other programs to make up for all that discrimination. When are we finally absolved for something that our ancestors did a long time ago?