Naturally, I can't open-source any private business code. But if I'm doing my job right, I'm building libraries and reusable code that isn't tightly coupled to business logic -- and there's no good reason to avoid throwing all of that on github, and have it be used by as many people as possible.
Of course there are going to be companies that aren't onboard with the idea of open-source. It's taken a lot of places a lot of time to get to that stage. But are those the places you really want to work?