I guess someone could, but it ends up being more practical to use a translation layer (which inevitably ends up having at least some overhead) on a niche platfo

I guess someone could, but it ends up being more practical to use a translation layer (which inevitably ends up having at least some overhead) on a niche platform, while being able to wring every last possible bit of performance using Vulkan on the other platforms. Hence MoltenVK and not VulkMetal. Or maybe it's just about the momentum, which Metal doesn't have. Note that WebGPU has been heavily inspired by Metal (AFAIK), and it uses translation layers on top of a number of APIs (whatever is available on the system). And it's not limited to browsers, despite what its name might suggest. So it seems like your best bet to try something Metal-like that works across platforms.