OpenGL is a dead end - while vendors (including Apple) still support their respective GL stacks, there is not really active investment anymore either in those stacks or in the standards.

Metal came out years before Vulkan, and Apple has tight integration between the graphics API and their underlying first-party graphics hardware designs. If Apple did have first party support for Vulkan, it would basically be MoltenVK anyway. Apple tends to push anything which isn't a first party framework to be third-party sourced and bundled as much as they can. They likely think MoltenVK as a third party library is the best scenario for Vulkan support.