I did not work for Apple but this article describes exactly what I was thinking. Apple has very waterfall cycle. Everything is planned year after year to be released for the September event. When the deadline approach teams realized that are behind and they need to overwork. And when people get tired and under pressure, they usually introduce even more bugs. The article also nails the solution: a more Agile workflow.…
Non-breaking API additions and userspace features which do not impact API can be rolled out throughout the year though. There are also changes to private API throughout the year since the Apple apps using them are distributed with that operating system release.
Generally, API grow up as private API that get adopted by other Apple apps, then may get promoted to public once they are stable and maintainable.
IMHO, the big issue with iOS 13 and Catalina this year was that there were several huge developer features and internal system refactoring which were maturing during beta. This slows down everyone who is dependent upon those features. The solution there is to require public API to have a higher level of maturity before that beta 1 release.
I disagree that Apple is waterfall, because their release dates are fixed, and they will change what will be delivered to make that date. I don't know if a twice annual major feature releases (and corresponding WWDC summer and WWDC winter) fixes anything, or if it just makes the life of third party developers harder.