Why is Apple still dead set on having their own special graphics API? Surely it would be better for everyone (even them) if they just adopted Vulkan like literally every other platform?
What other platforms? The only ones I see adopting Vulkan are Desktop Linux and some newer Androids. The first effectively doesn't even matter.
This isn't really something that helps or hinders locking consumers into their ecosystem.
The games consoles don’t use Vulkan either. It’s a niche API.
So it's more a case of they built the API for themselves first and then made it available for developers.
They also have power consumption and mobile performance as being their highest priorities which may not align with Vulkans.
It is not in any ways better than Metal. And when you design your own Hardware, it makes much sense to own your API design for maximum performance.