Any individual that provides free labor cannot survive off of said free labor. He must work for money to survive or get donations from someone who earned that money from incentive based labor in order to even buy the food he needs to exist as a living human being. Much of the time that labor is actually closed source.
This is a logistical reality. A lot of open source advocates are unable to get their brains out of the whole mentality that open source literally cannot exist without incentive based software supporting it. Who pays for GitHub to exist? Who pays for the food swes eat? I just code for open source all day and money falls out of the sky.
My smart friend says there are jobs that pay you to work on open source exclusively. Smart guy. In this case you follow the money trail. How does that company get enough money to pay a guy to work exclusively on open source?