There are many, many systemic forms of gentle suppression that lead people to believe and internalize things that aren't true.
As a 39-year-old transwoman who has been in the industry, and who studied in STEM in the first place, I couldn't help but notice a lot of gender policing that subtly and not-so-subtly discouraged women from taking technical roles. There's a deeply-ingrained belief that "women aren't interested in programming" which leads to any woman who tries to be a programmer being judged way more harshly than her male peers, leading them to become discouraged and feel that they're not cut out for it.
Meanwhile, my own gender identity was always being put into question because of my love for programming and video games; the fact I was good at programming and wanted to make games made me automatically "not a real woman." And I saw this happen to my cisgender female peers as well! Any woman interviewing at a studio would automatically be assumed to be an artist, or "just" a tools programmer, or whatever, and after working 3x as hard to prove herself she'd be told that she's "almost good enough" to be "one of the guys."
This all starts very young, too. There's a lot of background radiation that's easy to internalize and difficult to separate out. And this goes both ways as well - look at the huge gender disparity in "feminine" vs. "masculine" roles, even in the same field; dentists are men, hygienists are women; doctors are men, nurses are women; game programmers are men, game artists are women. It goes on and on, and all of it is due to a lifetime of self-reinforcing beliefs.
Biases are hard to look past, especially when it's hard to even tell where those biases are in the first place.
Fortunately, things are (slowly) changing. We're seeing a lot more encouragement of kids of all genders going into whatever fields they are interested in, and attitudes are shifting away from a gender-essentialist worldview.