They actually have a department to write the documentation. The problem is that it’s a separate department from the people who design the libraries themselves, which means instead of having high quality documentation throughout, you only get the documentation that the doc team had time to write. It’s why most of the APIs are minimal at best in terms of documents.

While the level of documentation that exists is usually pretty good, the sad fact is that most documentation doesn’t exist, and it’s entirely due to the fact that they are handled by developers/documentors separately. If you had developers documenting in-situ in the code, instead of WWDC slides, then it would be a lot better.