Or they could actually properly support and maintain OpenGL (or OpenGL ES, even). Or they could offer Vulkan. They don't even need to be native implementations, they could be Metal wrappers maintained by Apple. It doesn't take much.
On Windows you've been able to use OpenGL if you want, literally forever, because there was always a baseline version of it available with the OS (and then if you installed video drivers, you got a GOOD version of OpenGL.) Now you have the option of native Vulkan if you have drivers too. All of this is maintained collaboratively by Microsoft and the GPU vendors' driver teams. You don't even need a GPU, Windows has shipped software rasterizers for Direct3D literally forever as well (and the modern software rasterizer, WARP, is actually quite fast)
And on Linux you can either use OpenGL (natively), Vulkan (natively), or Direct3D (via Proton), and all three perform great and are reliable, maintained collaboratively by the Linux community and GPU vendors' driver teams.
Meanwhile on Mac your option is to write your own custom Metal backend, or hope that an emulation layer you get from a third party like MoltenVK will work good enough for your purposes and won't break when your end users install an OS X update, since Apple is happy to break backwards compatibility for any reason any time they want. If the build of MoltenVK your app ships with breaks you need to integrate a new version and ship updates to everyone.
On Windows you've been able to use OpenGL if you want, literally forever, because there was always a baseline version of it available with the OS (and then if you installed video drivers, you got a GOOD version of OpenGL.) Now you have the option of native Vulkan if you have drivers too. All of this is maintained collaboratively by Microsoft and the GPU vendors' driver teams. You don't even need a GPU, Windows has shipped software rasterizers for Direct3D literally forever as well (and the modern software rasterizer, WARP, is actually quite fast)
And on Linux you can either use OpenGL (natively), Vulkan (natively), or Direct3D (via Proton), and all three perform great and are reliable, maintained collaboratively by the Linux community and GPU vendors' driver teams.
Meanwhile on Mac your option is to write your own custom Metal backend, or hope that an emulation layer you get from a third party like MoltenVK will work good enough for your purposes and won't break when your end users install an OS X update, since Apple is happy to break backwards compatibility for any reason any time they want. If the build of MoltenVK your app ships with breaks you need to integrate a new version and ship updates to everyone.