Contributing to open source means a lot of evening hours spent in front of a computer, that I could spend doing something else - spending time with my wife and daughter, playing music, sports, whatever. Of course, open source is based on passion and that's what drives me.
But the point is that the time spent coding for open source is *unbilled*, meaning I'm not directly making a living from it. I might be re-using open source software for my professional work, but typically the code that I write for paying customers or for my employer is not contributed back to the community.