Yeah but Windows is a more stable api to develop against than Linux (at least when it comes to stuff that games need to do) - it doesn't feel "pure", but pragmatically it's much better as a game developer to just make sure the Windows version works with proton than it is to develop a native Linux version that's liable to break the second you stop maintaining it.

As someone once said it best, Win32 is the only stable ABI on Linux: https://blog.hiler.eu/win32-the-only-stable-abi/