The problem with an enlightened and benevolent ruler is that even if he really is that good (and history uncovers a lot of dirt even for rulers popularly known as enlightened and benevolent) what do you do when they die or decide to quit ? Their children could be insane psychopats unfit to rule (as indeed often demonstrated in history) or there would be a civil war with various factions trying to fill the power vacuum.

With all its faults democracy has these edge cases handled and makes the elected group of rukers much more acountable with mens to get ridd of they if they turn out to be too unfit to rule.