If most developers are targeting iOS and macOS for gaming, it make sense to use game engines, if I understand some developers had shared their discussion in /r/vulkan found it was harder to code in Vulkan than Metal 2?
There are applications beyond gaming, however, and it is still kind of a drag that OpenGL is still the only de facto cross platform API in the space, and that it has miserable support on macOS.
Are you aware of any project that is under development right now? I know it's theoretically possible, but I haven't seen any implementation yet.
Vulkan is not very nice to work with. It's.. ok.
Metal is nicer
Khronos APIs keep being stuck on their C API definitions, it was NVidia effort to create a C++ wrapper that eventually made it into Vulkan, yet the shading language is pretty bare bones.
Similarly with Metal Compute shaders, Khronos had to see CUDAs and C++AMP adoption to see that many wanted something more productive than OpenCL C. However not many boards do support OpenCL 2.2 with SYCL.
Metal is a nice balance.