Imagine if the OS let you browse all the app data, see all the relations (that you were authorized to see), and write tools to operate over it. That would be a most Open system, compared to everything we have which is essentially walled off to us.
Either you somehow enforce a high degree of uniformity, which implies a pretty serious lack of flexibility, or everybody gets to make their own decisions and then everybody makes different ones. And the second one seems better as long as the individualized thing they're doing is sufficiently well documented.
Opening up application data to OS tools, opens up a whole new world of opportunities (and hazards) for app developers.