Which leads to the solution: Quote a lot more for Apple development or don't develop for Apple, and as soon as the stream of app updates and releases dries up, Apple might react. But not before.
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.
However, apple is a business, and leaving out docs makes business sense and is an easy and straightforward explanation.
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.
They should have all the resources in the world to recruit people that have proven to write good documentation. If open-source projects run by volunteers can have excellent documentation (e.g. Vue), why can't Apple?
Better docs mean a better developer experience which means more people want to (and are able to) develop apps for iOS which increases the value of their platform. It looks like a no-brainer to me to invest some resources into this to improve the current state of affairs.
I must be missing something here...