Technically so doesn't Windows, nor game consoles in spite of urban myths (Switch being the exception).
Windows Vulkan and OpenGL drivers exist, because Microsot still hasn't removed the ICD plugin interface from the OS, which is used by GPU vendors themselves, not Microsoft, to provide drivers on Windows for VUlkan and OpenGL.
Likewise, Valve could have use MoltenVK if they actually wanted to.
Not "likewise". MoltenVK is a compatibility shim that wraps Vulkan to Metal, not a true hardware interface. This has impacts on performance and compatibility, some features straight up don't work because Metal doesn't provide an equivalent. Given the apparent legal entanglements between Kronos Group and Apple re: Vulkan, it seems unlikely that true Vulkan support on Apple hardware will ever happen outside of Linux.
Valve didn't bother with Metal because Apple are hostile to their business model, they've given up dedicating serious rescources to the platform and at this point Steam only exists on MacOS out of inertia.
The venodor provided OpenGL and Vulkan drivers on Windows are not shims over DirectX and are instead full-featured implementations of those APIs for the vendor hardware without weird limitations that a shim brings. So yes, the situation on Windows is better, by miles.
Windows Vulkan and OpenGL drivers exist, because Microsot still hasn't removed the ICD plugin interface from the OS, which is used by GPU vendors themselves, not Microsoft, to provide drivers on Windows for VUlkan and OpenGL.
Likewise, Valve could have use MoltenVK if they actually wanted to.