It's simple policy. Apple do their own dev and don't use third party companies for the development. This is for a variety of reasons including providing a tailored platform to their specifications, and meeting the level of secrecy required internally.
Whether you agree or not with the policy, it's not one motivated by lock in.
And again it's not a matter of being cheap. It's a matter of perspective. Apple believe metal is worth using and that it delivers needed performance, optimization and features for their stack. Apple also believe in very straightforward developer stories within their own platform. Vulkan doesn't provide value above and beyond what metal does from apples perspective, and it complicates their developer story.
That's my point too - their perspective is lock-in. Not just in graphics APIs but in many things. So this issue falls into the common picture with many other similar ones.
I.e. Apple could easily implement it. They don't for political reasons, and I don't buy any lame excuses like "that's better for developers" and such.
You seem certain that preventing people from hastily porting Android games was NOT a motivation for Apple.
But they have explicitly said in the past: they really prefer we not put cross platform apps on the App Store.
I don’t know why you won’t consider the odds that Apple is trying to hamper cross platform apps with Metal deliberately as a policy when they’ve specifically said that’s their strategic goal.
Theorizing about non malicious motives doesn't change the facts - the result is lock-in. And when it consistently happens all the time for supposedly "other reasons", excuses about it not being done for the sake of lock-in stop being even a tiny bit convincing. To put it differently - who cares about motives at this point, when the result is a nasty lock-in which Apple refuses to remove.
Whether you agree or not with the policy, it's not one motivated by lock in.
And again it's not a matter of being cheap. It's a matter of perspective. Apple believe metal is worth using and that it delivers needed performance, optimization and features for their stack. Apple also believe in very straightforward developer stories within their own platform. Vulkan doesn't provide value above and beyond what metal does from apples perspective, and it complicates their developer story.