Many games ported to Linux use an old version of OpenGL that's still compatible with Macs. You can't even get AA in Civ V on Linux. Not to mention what a headache installing closed source video card drivers is on linux. I broke things plenty of times trying to do that in my noob days. I'm sorry but it's just nowhere close. Less games and many of the games available perform worse. Vulkan could have changed that but it hasn't. I'm not seeing massive amounts of support for it yet.
Not true.
Apple has stalled development of OGL on their platform, titles are not ported from MacOS. In many cases OGL is being depreciated in favor of Vulkan and Linux supports OGL as well as Vulkan 100%. Linux has overtaken MacOS as the gaming platform of choice second to Windows.
Installing Nvidia drivers via the PPA method is actually easier than the same process under Windows, furthermore they keep themselves updated. Running Nvidia drivers under Linux, FXAA under Civ V is able to be forced via drivers - Not that the lack of AA under Civ V makes that much of a difference considering the title.
Vulkan support is enjoying adoption pretty much in line with DX12, DX needs to die in a housefire for the betterment of gaming in general. As stated, I'm gaming under Linux,~80% of my library is supported, performance is great, sometimes better than native Windows considering Vulkan and no resource sucking anti virus needed.