None can force anyone to use something
Another evidence of Metal unpopularity compared to game engines this is from Apple directly
According to Apple, more than 148,000 applications use Metal directly, and 1.7 million use it through high-level frameworks, as of June 2017.[7] Presumably, most of these are applications and games on iOS. Notable macOS games using Metal for rendering are listed below.
This is even lower than 10% of all MacOS and iOS apps rely on Metal directly. Imagine how much worse the OpenGL is.
And Apple owns the largest game market in the world right now (see iOS revenues). So for them Graphics is a huge deal as iOS devices are mostly entertainment devices.
Obviously Blender cannot rely on existing game engine for a ton of reason but it puts in perspective how really important technologies like Vulkan, Metal and Direct3rd are. Even though they fuel the world of graphics for pretty much anything out there, developers want to do as little as possible with them and rarely deal with them directly.
This in turn means lack of documentation, third party support, community etc. And if Metal is so unpopular imagine a port of Vulkan on MacOS.