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…
hell, most programmers would probably take a pay cut to work in a "cool" language. i'm willing to wager that C/C++/COBOL/java mean salaries are a standard deviation higher than haskell/elixir/clojure salaries. why? many reasons but i bet that companies know that they can use programmers' passion for mastery against them. leave your boring C++ slog job to "hack in Haskell!"
i'm guessing many of these guys are just clueless when it comes to how potentially valuable they are as individuals and what that's worth, while they put up a $10/mo patreon that 6 people use. i'm guessing most engineers are severely underpaid when it comes to value added, they consistently fall for the okie-doke's of "we get to type stuff into a computer and learn and oh wow, i ought to be so grateful for my six figure salary", yet they're putting entire swathes of the economy out of work for their corporate overlords.
it's tough to convince management you're worth a lot of money if you're willing to give away your precious time and resources freely.