100% this, Apple wants complete control over the distribution and tooling used on their platforms. While it does provide advantages for their hardware development I think it just ties back to them wanting 30% of any application or gaming revenue.
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.
PC games that could be ported to Mac and published on the App Store are less likely to do so since there's a lack of support from Apple's side. They care more about mobile games it seems.
To be fair, 30% is what Valve as a distributor charges (glass houses and all that). Valve the game developer is a hobby project subsidised by Valve the distributor.
