A) Many games don't use these engines. The trend has actually been going in the direction of less custom engines, but a significant amount of games still do.

B) Games might not have to deal with macOS if they're using an engine, but the engine themselves still do. This limits what engines can do to the least common denominator. And we all know Apple isn't the fastest when it comes to new standards or features. In the graphics world, that comes in the form of DirectX12 vendor-specific extensions, with Vulkan vendor-specific extensions a few months later, and then Metal support whenever Apple feels like copying it.

Someone else in this thread linked an example about Metal lacking support for a type of atomic primitive that prevented the decoupled loopback algorithm from running on Metal. That means the author couldn't use WebGPU to implement the algorithm, and had to make an implementation in Vulkan and Metal separately (iirc all the details of this correctly, you get the general idea). And it _wasn't_ clear that they couldn't use WebGPU to begin with. The WebGPU spec had to actually be clarified based on the author's investigation. Engines and other APIs wouldn't have to deal with this if Apple just supported Vulkan.