While I wouldn’t want this to be a general statement: platform-owned apps can be really terrible. Why? Because they don’t permit level-playing field competition between their own apps and third party apps on their platform. And that lack of competition is costly; maybe even for the platforms themselves.
It is precisely when a use case is ill defined that you want to encourage platform experimentation and innovation.
I say all of this to highlight a theme but also to argue that for some platforms the cost is worse than others. Sadly, it is hard to come up with a convincing business rationale that allows this state of affairs to continue and certainly impossible to come up with one that squares with the seeming damage done to user experience.
iTunes and Twitter are indeed good examples of this. (And there is no reasonable business argument against modularizing the user interface of the former.)