Most of the issue revolves around GPU drivers, which in theory valve is not responsible for, but they have hired devs to work on the open source AMD drivers. Even then, most devs don't optimize for OpenGL. That's again on the dev, and not valve, unless valve is going to provide devs to every game developer out there to provide linux support. Vulkan (hopefully) would resolve that, as optimizing vulkan builds makes games run better everywhere.
Of course they aren't putting ALL their eggs in this basket. ~90% of their revenue comes from windows, so they should be putting most of their effort into windows support. You can't lose your current customers while trying to make the future better. The fact that they are in fact fixing things and hiring driver devs is a great sign. And I believe they provided the rocket league dev too?
Either way, progress is coming, slow and steady. linux as a primary gaming platform works just fine for me, thanks to valve.