Some people say that Celestia and Luna are useless. That they lounge in the castle while delegating all their responsibilities to hard working, underappreciated, exceptionally well-read students. That even when faced directly with threats to kingdom and country, they fold like cheap origami and wait to be rescued.
These people are correct. But there's a very good reason for it!
[Rated Teen due to a dark scene early on to set some backstory, and occasional comedic death]