FWIW Metal is actually easier to use than Vulkan in my opinion, as Vulkan is kind of designed to be super flexible and doesn't have as much niceties in it. Either way, OpenGL was simply too high level to be exposed as the direct API of the drivers. It's much better to have a lower level API like Vulkan as the base layer, and then build something like OpenGL on top of Vulkan instead. It maps much better to how GPU hardware works this way. There's a reason why we have a concept of software layers.
It's also not quite true that everyone uses Unity and Unreal. Just look at the Game of the Year nominees from The Game Award 2023. All 6 of them were built using in-house game engines. Among indies there are also still some amount of developers who develop their own engines (e.g. Hades), but it's true that the majority of them will just use an off-the-shelf one.
Metal is probably the most streamlined and easiest to use GPU API right now. It's compact, adapts to your needs, and can be intuitively understood by anyone with basic C++ knowledge.
It's also not quite true that everyone uses Unity and Unreal. Just look at the Game of the Year nominees from The Game Award 2023. All 6 of them were built using in-house game engines. Among indies there are also still some amount of developers who develop their own engines (e.g. Hades), but it's true that the majority of them will just use an off-the-shelf one.