@AlmondMagnum
The main reason is because historically, it was the nobles in the kingdom that caused problems for the people, not the king himself, even despotic kings rarely were cruel to the commoners themselves, that said both nobles and kings cruelty were actually far less common than history paints them, but in a world that is fond of republics and democracies, monarchy is anathema, similar to dictatorship's viewpoints, as monarchism is literally the opposite of republicanism, as such neither likes the other (this is part of the reason ww1 was so absolute, it was monarchism vs republicanism), funnily enough, monarchistic nations in the past painted republics and democracies with the brush we paint monarchy's with today, so i guess it's just the cycle of history, oh and alot of times, nobles were part of a noble senate under the king, and considering julius ceasar and england's histories with their senate/parliament, i think history speaks for itself.
The nobles good, emperor/king bad is however, far more common in korean manhwa, so part of it may be cultural.