Because Dark Knight, Astrologian and Machinist are, as said, Extra Jobs, they don't have a class forming their base, an important feature by which to share abilities from. And rogue isn't cross class for any Jobs, an equally important feature if other Jobs are to pool from it's abilities. They are, literally, their own things. There's nothing you can do about the Extra Jobs as, technically, they are completely quest-based. In fact, all Jobs are quest-based and there's not one precedent, not even in the classes with their two quest-based abilities, where quest-based abilities are cross class. Should this change? Well, first, they'd need to change the class system, but in order to do that they'd have to divorce Jobs from classes. Once they do that, then they'd be free to move on classes and maybe change the cross class system. But this would all be a not all that small a change, and would have to be approached very carefully, else they break the game.