As a long-time professional game engine programmer, it is hard for me to see consternation over things like this, and avoid judging it as mainly ignorance. The amount of code in an engine that needs to touch the graphics API is tiny. A handful of methods for device init, filling buffers, uploading shaders, setting state, and saying "draw!" All of the graphics API code can easily fit in one medium-sized source file. As a proportion of the whole engine, it's very small. As a proportion of the whole game or app, it's negligible. It's also boilerplate. There are only so many ways to copy a stream of structured data into a buffer.

Legacy software, blah, blah, blah. No legacy software runs forever, and least of all on Apple platforms. Who cares.