Well, hardcore batshit is difficult to argue with and I'm just a thick-as-shit republican, but here goes:
The institution of monarchy, like fascism, is anti-democratic. You could put the most saintly and virtuous human being in a palace and fawn, scrape and bow to them because of who gave birth to them: they are making huge decisions for huge numbers of people who never asked them to do that, people who have been coerced or acquiesced because they had no choice. The people have no power, the monarch has it all, and so they will always be open to corruption.
They can do whatever they like. And that's what they do.