The real question is why Metal instead of Vulkan.
Besides, while the boilerplate for Hello Triangle is indeed very big, it makes a lot of stuff easier to express on top of it. GL may have a much smaller Hello Triangle, but you have a lot less control over it, way too much stuff happens under the hood. And regardless, if all you want is to animate a web page with fish on an aquarium you really should be using GL instead of Vulkan.