It struck me as odd actually, as I have always believed this to be a beautifully optimized game (it still use). I've used the Linux build on my laptop (with propritary NVIDIA) and runs exactly like the Windows version.
On Steam Deck under Proton I am now using the High preset, then manually maxed out environment quality, weather quality and population and still remains at fixed 60.
Windows integrates only very early versions of OpenGL (just kept from the 9x releases). Any modern release is implemented by the driver of your 3D accellerator/video card.
OpenGL on Windows has always been kinda of a disaster (NVIDIA's a little less, but AMD and Intel's are just abysmal), DirectX support being more developed is night and day.
Linux is pretty much OpenGL's home. But a lot of applications just are not optimized well enough to show it. Vulkan being faster is just because the software using it have cleaner codebases for being newer.