Because Valve assume majority of users are using Ubuntu or derivatives. Which doesn't contradict what I said above. RedHat developers contribute a lot to Mesa. Canonical isn't exactly known to do that.
Something yes, but not improving it directly. Besides, Ubuntu proper isn't even the most used distro probably. Mint is likely more used.
My point is, those who work on Mesa (OpenGL / Vulkan) and Linux graphics stack have way more direct impact of Linux gaming (they are fixing bugs that affect games, improve performance, add new functionality and so on).