I want to work with the best people. Many of the best people are female. Some of these women don't want to work an an environment where they are the only woman and since they are excellent, they choose to work in places that already have female engineers.

Also, I generally prefer to work in places with a diverse mix of people as well. Given two job opportunities that are roughly equal, I'll choose the more diverse one because it's more interesting.

A while ago, I got an MBA. MBA classes are extremely collaborative and a lot of the learning comes from the other students, rather than the lecturer. Most good schools very deliberately to ensure that the student body is as diverse as possible because otherwise learning is stifled: if ALL the students are investment bankers, then the class doesn't really learn much about other businesses. I see work environments similarly.