This isn't clear-cut nowadays (especially if you avoid Ubuntu). Linux and Windows trade blows in terms of performance. OpenGL, Vulkan, DX11, and DX12 are all supported on Linux. Some features are a work in progress (RT on AMD for example), NVIDIA are cleaning up their act and their Linux experience matches their Windows experience (and they have full API coverage). Windows only holds dominance through anti-cheat support.

I would say that PC is still the platform for gaming.

I think you have this reversed - it's application (game) level backwards compatibility leading to large market sizes of people who have gaming PCs, which leads to a cycle of dependency which Windows just happens to be a required part. SteamOS, Wine, and similar are attempts to break Windows out of this cycle.

Hardware support is similar - vendors go after the biggest market. Also PC gamers are used to installing kernel drivers from random third parties, whereas other OS vendors usually want drivers to be integrated in the main release.

The Steam Deck and other developments (Proton/DXVK, Nvidia kernel drivers, EAC/Battleye AC support) have weakened that position, though.

Linux is still only 1.2% of PC gaming, but the idea of that increasing isn't as bananas as it was in 2015 before any of these developments were announced/released.