Linux developers aren't on a nine-to-five salary like the Redmond and Cupertino crews.
At least for the kernel, this is no longer true - and hasn't been true for quite a while. Most *kernel* contributions come from people who are paid to contribute, including many who are paid by a business such as IBM or even Microsoft. The whole 'it's volunteers' isn't true for the kernel and quite a few of the more popular projects.
(This is a good thing.)
Now, I'm on the Ubuntu and, more specifically, Lubuntu teams. Sure, I get a fancy email address and the ability to speak on their behalf or other things, I'm absolutely unpaid. So, that end of things is still very much a labor of love. It'd probably be pretty easy to parlay my position with Lubuntu into a paid gig, however. I'm happily retired and not even remotely interested in turning it into a job.