I would guess the environment candidates are selected from influences these numbers significantly.
And you're right, I'd understand that in some of these environments, OSS isn't necessarily considered fun or relevant.
Then again it's all speculation.
Because I myself feel that this is such a good and reasonable way of hiring people, I wouldn't want to work for an employer that wouldn't let me do open source work.
I mean, it's also because I consider open source to be the public sphere, and I really do believe in software freedom. Maybe my professional metaphor would be the coder as a writer. Of course I want to see your published work.
Out of curiosity, what do you mean by this? What sorts of things do they all have in common?
Regardless, I actually think this paradigm works for entry level jobs at most companies. You don't have to know anything for your first year in most cases. You just have to be able to show up and follow instructions.
show me freebsd or linux kernel contribution on github, for example
I do understand what you're saying though but I don't think people are going to just search Github for your name on the off-chance that you're on there, I suspect that what most people do is look at the things that you've specifically mentioned in your CV.