I assume it's just that being self-deprecating or humble correlates with many traits that make you a good developer, so people with those traits are more likely

I assume it's just that being self-deprecating or humble correlates with many traits that make you a good developer, so people with those traits are more likely to end up in this career path and stick around in it. Just like being a sales person doesn't automatically make you overconfident, but being overconfident makes you a good sales person.