That the most common case, though, not the exception. You make a CV that highlights your accomplishments, and you make sure you're prepared to dive into technical details during discussions. The hiring company tries to put your through your paces, somehow or other. It's imperfect, but the GitHub profile approach is going to weed out a lot of candidates who you probably ought not to weed out.
By far the most interesting work I'm doing is for my current employer; and it's not OSS (and wouldn't make sense to open source).
In theory, I could cut back on my work-work, and spend more time on side projects; but I have a family, and other interests as well. I'd rather have good overlap between "interesting work" and "work I'm paid for" than sacrifice my time with my kids or my guitar so I can work on interesting technical problems I'm not paid to work on.
For some people this could work out well -- e.g., maybe the crucial and awesome component of your company's infrastructure that you're building can be open-sourced! -- but for lots of other people life simply won't line up that way, and building something serious (but a personal project) on GitHub in your spare time means you don't have spare time for anything else in life... I wouldn't demand that of myself, or of anyone I was hiring.