Do you expect Rails developers to have their own pet projects that they put a lot of time into? I agree it would be strange if no one had any forks, had never done a pull-request etc.
It's not about requiring everyone who works for us to be a 23 year old with no family or life who spends every night hacking on shit, just about a baseline standard for community engagement. Lack of Github account with some level of activity (even if that's just starred repos, forks, etc) is a pretty strong indication someone wouldn't meet that level of engagement, and so wouldn't be a good match.