If Valve's Employee Handbook is to be believed, they don't use a formal project structure with static teams. Instead each developer works on whatever project interests them, and one of Valve's current goals is to improve game performance on Linux/AMD by contributing to upstream open source projects.

Valve is as close as we've gotten to someone paying a bunch of industry veterans to contribute to open source. It's amazing what happens when all innovation isn't black-boxed in an internal repository and forgotten about.

I suspect that it's mostly that they're the only ones who care.

Valve directly profiting from all those games being available on their platform. Them helping out is a win-win-win situation.