Metal allows Apple to squeeze that extra performance out of their devices. They have full control over, and can implement whatever they need to deliver Apple Vision Pro for example.

With Vulkan, they would have to wait for a committee to approve required changes, and still they could not probably optimize it to match their GPU an CPU hardware profiles in such an efficient way.

And not least at all, to optimize the developer experience and tools. Apple GPU debugging tools are probably the best tools you get for graphics development debugging, and you get that only on macOS.

IMO Metal is a nicer API than Vulkan. At the same time, you can ask, why should Microsoft get to keep DirectX, and not just write a Vulkan driver ?