Discussion on: Why are there fewer female developers?

Your first and third points assume a cultural aspect that's very prevalent in many parts of the Western world. In essence, the biases that exist against specific genders in certain jobs (not just against women in STEM jobs, but also things such as biases against men in childcare or other people/emotion focused jobs) are a non-negligible part of why these types of gender inequality exist, but the very existence of these gender inequalities helps perpetuate those biases. It's essentially confirmation bias, but with a multigenerational feedback loop.

However, in other parts of the world, that bias either manifests in other ways, or the minorities there who the bias is skewed against are actively ignoring it and trying to get rid of it.

It's also important to consider though that being exposed to those biases as a child colors how one looks at their own life. Humans quite simply tend to subconsciously try to maintain what they view as the status quo. If you're a girl who's grown up exposed to the assumption that most engineers are guys, it's less likely that you'll consider going into engineering, though you probably would have to think a bit if asked why you didn't consider it (and would probably come up with some excuses other than "I'm not a guy."). If you pay attention, you can see the same kind of behavior in other minorities as well, even to the point in some cases of people essentially hiding behind their minority status as an excuse to not try and proactively improve their situation.