If it was the devs telling people to build it themselves, I could understand it, even if it's a bit rude, but it's often unrelated users on general forums. There's an attitude of 'If I can do it, then you should be able to', and a lack of understanding for people who can't.
I also wouldn't say being built on volunteer work is always an issue.
While maintainers can burn out (I.e. due to demand for new features without more resources/help), volunteering also means every contributor is self motivated to make the software good, not because they're extrinsically rewarded.
Bigger projects like Linux tend to be more supported by people with either a side job or getting directly paid to contribute to Linux by their employer, so its less of a problem for bigger projects
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While maintainers can burn out (I.e. due to demand for new features without more resources/help), volunteering also means every contributor is self motivated to make the software good, not because they're extrinsically rewarded.