Valve is making money on Proton (if indirectly), so they would very quickly get sued for it. It’s not that the copyright owners would want to stop them from distributing the library - just that they’d want a cut of Valve’s massive revenues. A community project, on the other hand, isn’t likely to be able to provide any revenue. So since it’s just redistributing runtimes that are already available for free and the only likely result is the project getting taken down, it’s not really worth doing.