If you're using a major engine, yeah, Metal's a non-issue. If you're rolling your own tech or using a smaller engine, a Metal port is a big pain. It's not easy to port to the modern, low level graphics APIs like Vulkan & Metal, so it's a deal breaker for some devs. The shaders are trivial - all the shader languages are pretty similar, so unless you're doing something with bleeding edge shader tech, it's usually easy to port them.
But yeah, the market size is the real issue. It's not a question of whether or not people can port to Metal. Even if you're using an engine that handles that part of the job, it's still usually not worth the time and effort to do the port.