Why is a naval captain senior to a naval commander?
Because this order of rank is part of the universally understood definitions of those two words. It’s an unalterable convention of the English language. Whoever says that a captain is lower than a commander, in any navy or fighter force, is simply and objectively in error. Note that an army captain is not the same as a naval or pilot captain; he equals a naval/pilot lieutenant, and this rank is inferior to a naval commander and naval lieutenant commander. A naval/pilot captain is equal to an army colonel.