My employer (not posting the name since I don't care to be directly googleable) de facto forbids open source work. In order to share essentially any code we're supposed to pre-clear it with our internal "open source council" by filing a jira ticket and asking permission. Each time, every time. They meet monthly, so you potentially wait up to 30 days. Of course the company realizes they only own our work that is directly related to our employment or that is done with company property. On the other hand, they have two in house attorneys, one of whom was a partner at a very high end law firm. So in practical terms, in any legal pissing match I'd lose, no matter the merits of the case, since I can't afford my own $600/hour IP attorney.
Which is a long way of explaining that many people have very high barriers to sharing code. So the only code I really have to share is written for my employer, and I obviously won't be sharing that. I think that having shared code on github is a poor estimator for quality of an employee.
It's a misconception that every employer who asks for a Github ID as part of a job application process is looking to use it as a way to screen applicants. For us, it's just a shortcut to see whether someone can code. If they don't have a Github (or bitbucket, or technical blog, or whatever else is Googleable or that a candidate wants to provide us), it just means that I have to spend extra time during the phone screen or in-person interview on their coding abilities.
Which is a long way of explaining that many people have very high barriers to sharing code. So the only code I really have to share is written for my employer, and I obviously won't be sharing that. I think that having shared code on github is a poor estimator for quality of an employee.