It's easier for them to only have one API, they still have to write and maintain drivers, and Metal predates Vulkan. Microsoft also requires all graphics driver

It's easier for them to only have one API, they still have to write and maintain drivers, and Metal predates Vulkan. Microsoft also requires all graphics drivers to support DirectX, and it's up to the GPU maker to support OpenGL or Vulkan. Metal is far from half-baked. After working with OpenGL for years, the stability of Metal is a breath of fresh air. No GPU-specific workarounds, great performance with unified memory, and easy interop between compute and graphics.