Other problems in the dev culture accentuate this problem even further. It seems there is a complete lack of dependency graph between the software stacks; apple engineers who opt to rewrite a piece of software or functionality have no documentation or idea how the software is used or was originally intended to be used, be it inside Apple or otherwise, often causing serious design-level bugs, often resulting in major rollbacks. The lack of any QA work is also very apparent, especially in recent years, where simple bugs and glitches are allowed to go through, let alone major rewrites. The only tool to counter this is radar/feedback assistant, which are tools well documented in their inefficiency from current and ex Apple developers themselves.
I don't think this should solely fall on Apple developers, of course. As is often the case, such inherent problems in culture are due to lackluster management. This has been hinted at by many ex Apple engineers.