Not only gaming is a thing on Mac, more than on GNU/Linux or even BSD combined, it is also a thing on iPhone, iPad and Apple TV, all with Metal support.
Also Apple's augmented reality is based on Metal, and they had Valve on stage praising it at WWDC, with native support on SteamVR SDK.
Finally, many of the frameworks that made use OpenGL, including the window manager, are now working on top of Metal.
Apple is using Metal because it's a modest improvement over OpenGL (in some areas; it's a regression in others) and, more importantly, their OpenGL implementation is extremely buggy.
You haven't said anything about Vulkan vs. Metal, and you can't, because Vulkan is a better API for the reasons I described upthread.
Full disclosure: I have colleagues and friends on the Khronos standards group, and I don't like seeing their work bashed without specific technical reasons.
Also Apple's augmented reality is based on Metal, and they had Valve on stage praising it at WWDC, with native support on SteamVR SDK.
Finally, many of the frameworks that made use OpenGL, including the window manager, are now working on top of Metal.