They are discouraged from birth from taking an interest in STEM related activities and subjects and the ones that persist and get to work in the industry are dealing with male colleagues who are being paid more than them for the same work.
Never came across this, but then again, I'm from a family of mathematicians/nerds who all work in STEM (as do I).
Not a good enough answer to the question. In fact, the question is "why do women/girls 'freely' choose not to go into STEM when they have the aptitude and it would give them access to well paid careers?" What is influencing their "free" choice, and are we happy with those influences?
Maybe it is a free choice? I went to a school where everyone was encouraged in each subject equally, top maths/science sets 50/50 boys/girls.
Come A-levels, a lot more boys pick physics/economics/CompSci, a lot more girls picking art/english/psychology/business.
Maybe they WANT to? No-one is standing at the Google offices banishing all women and telling them they can't code, that's bullshit. Would you all really be so outraged if a lot of women, under no pressure, find non-tech subjects more interesting?