My wife makes HALF of what I make at the SAME employer. She does semiconductor and microelectronics back-end process engineering. I do RF/Microwave circuit design.
She transferred divisions a couple of years ago. In her old division (mine) there were TWO female engineers out of probably 100.
It's not constrained to woman though, as 80% of those who graduate with an engineering degree are no longer practicing engineering by 40 years old.
She wants to quit every day. We can easily live off my salary, but the additional salary is good for college savings (2 kids) and retirement savings.
She want's to quit and quilt full time; yes quilt. She is really good at that, applying all the engineering knowledge to CNC quilting, tricky geometry on piecing, etc.
May have to do with woman, in general, being more risk averse than men. Also less agressive for self promotion. At my employer, you must promote yourself and make your argument in front of a review panel.