Apple for better and worse wants to be in control of everything. They want to control the hardware, and the software, and they dont want to rely on other sources to do it. They could use vulkan, but then they' have to share and thats not what they want.

In terms of gaming they make way more money on mobile games than the traditional gaming market does anyway, and in terms of production and commercial apps they just port over to metal.

Personally not for me, but it's what they are and I dont expect apple to change