Why can't a designated initializer call a secondary initializer in its base class?

According to the documentation, a class's designated initializer in Objective-C must call the designated initializer of its base class.

Another rule is that secondary initializers must call the designated initializer of their own class.

But if the second rule is followed, why can't a designated initializer call a secondary initializer in its base class? This base secondary initializer will ultimately call its own level's D.I., so the object will still be properly initialized, right?

The difference seems to be who chooses defaults for missing variables -- you or your base class.