I can also do handwaving culture arguments like apple design in a minimalist fashion, so everything that can be left out will be, including documentation. But I don't like those, I hate to diagnose cultures I only read about.
However, apple is a business, and leaving out docs makes business sense and is an easy and straightforward explanation.
They spend a lot of money on things like Swift Playgrounds, though. They have a whole developer outreach and training program: https://developer.apple.com/learn/curriculum/
It's a puzzling omission, given that building App Store revenue is such a priority, and they feed the supply side in a number of other ways. I don't think they're doing it on purpose, they're just bad at it.
iOS is Apple's bread and butter, and they make good money off those developers. It's in their cynical self-interest to spend the money to have the best documentation in the game, and it would even pay off inside Apple, with better APIs and better resources for their in-house teams to develop against.
It seems like a genuine institutional dysfunction. Apple has a culture of secrecy (which is necessary) and a result of this is deep siloing of teams. How this leads to documentation being such an afterthought is murky, but I suspect that's the cause, rather than merely being complacent about their walled garden.