Some other commenters in this thread have pointed out that nobody is forcing open source developers to work on the contributions that they make, and that is strictly speaking true. But in the culture of software development, there does seem to be an ambient message often repeated or hinted at, that it is good to "give back" to open source by contributing and that it is virtuous to do so. I now am starting to rethink…
Money is only a fraction of motivation for many people. Many do open source to make the world a better or more efficient place and reduce barriers to entry.