They don't have to develop it. They just have to not prevent it from existing. If theirs is genuinely better, it will get used. But Apple is too scared to openly compete so they lock down and artificially limit competition.
Anyone can implement the opencl and OpenGL APIs on top of metal. Apple isn’t preventing that.
But native OpenGL requires apple writing OpenGL implementations for their hardware. It requires apple spending engineering time implementing misfeatures from OpenGL and opencl in order to support APIs that are only every considered the backup API.
What about Vulkan? They’d need to implement the same crummy APIs and semantics (that is a bunch of kernel and similar work) for an API that NVIDIA and AMD were both uninterested in.
And again, anyone could have made a Vulkan emulation layer on metal.
