The answer is pretty simple: Apple doesn't care about gaming.

This goes back to Steve Jobs who didn't like gaming and apparently saw it as a waste of time. John Carmack mentioned this from his time working with Jobs.

Apple is now a leading gaming company -- not because they want to be, but because they're in a billion pockets and lots of those people enjoy gaming.

The reason the tooling and developer experience sucks is because Apple doesn't have an advocate for gaming at the VP or C-suite level.