Unfortunely Valve failed to make native Linux gaming a reality, not even game studios targeting Android NDK bother, which has the same 3D and audio APIs as GNU/Linux.
Who cares? What would that actually achieve and how would they have practically achieved it anyway? Use their store platform to force or coerce developers? Hold a gun to developers heads?
Valve don't owe anyone shit, neither did PC compatible BIOS manufacturers, nor anyone else who creates a clean room implementation of a pre-existing API. Getting Windows software working outside of Windows is a net good for consumers and developers.
Do you think Valve and Gabe will be around forever?
Proton works by wrapping Windows calls to Linux equivalents, which have been improving and becoming more robust as a result of this work. If the Windows game ecosystem collapses (How? When? It's literally never been more popular) then those equivalent APIs can be targeted instead. Meanwhile, the absolutely massive PC back catalogue, the platform's greatest strength, remains playable.
Where's the downside?
3D was also different (OpenGL ES vs OpenGL mess), only now it's starting to become kinda the same with Vulkan.
Do you need a tutorial?
While Mesa has OpenGL ES it wasn't always available (for example not all drivers are using Mesa).