There is such a thing as open source project with "pull requests welcome" in readme and whole elaborate "how to contribute" wiki chapter. Except when you follow that wiki to the T, it still ends up ignored because no one is reading pull requests. It is not that these contributions are going out of nowhere, from clueless people who dont realized maintainers dont want pull requests. Pretty often maintenners wanted pull requests, made sure to promote the option and then found themselves unable/unwilling to merge them in.
I am not saying projects must merge in pull requests. I am saying they should not have "please contribute it is welcome" messages in their readmes.
And on second plan, there are those campaigns to make people contribute. And shaming of companies/people for freeloading if they dont contribute - especially here on HN. Again, if then people conclude that contribution is something expected, it is not only their own fault.