OpenGL-on-Vulkan compat layers aren't magic. For them to support a given OpenGL feature, an equivalent feature must be supported by the Vulkan driver (often as an extension). That means you can't just implement a baseline Vulkan driver and get OGL 4.6 support for free, you must put in the work to implement all the OGL 4.6 features in your Vulkan driver if you want MESA to translate OGL 4.6 to Vulkan for you.
Plus, this isn't Alyssa's first reverse engineering + OpenGL driver project. I don't know the details but I'd imagine it's much easier and quicker to implement a driver for an API you're used to making drivers for, than to implement a driver for an API you aren't.
Plus, this isn't Alyssa's first reverse engineering + OpenGL driver project. I don't know the details but I'd imagine it's much easier and quicker to implement a driver for an API you're used to making drivers for, than to implement a driver for an API you aren't.