See this is not really true. For one thing, at scale democracies do not perform like at the local level. When voting for your Mayor, County Board and so on, you can make informed choices based upon the people involved as they are quite literally your neighbors. Unless you live in a massive bug hive with millions of people of course.
At the national level however you don't vote along these lines. You vote along a meta line that is easily controlled and directed. National government cannot function as a direct democracy because the nation as a whole is made up of so many people, many of them stupid and with competing needs and desires, that an Oligarchic control is not just required to manage the situation its necessary. Dynastic political families emerge naturally from this, as the ruling elites tighten their grip on power simply by the nature of how power works. Once in power, patronage can be established, networks built in, laws changed, and the entire appratus of the State turned towards the preservation of the entrenched power structure.
There is no functional difference in terms of ability, choice, or even inherited positions of power between a Nation wide Monarchy and a Nation wide Republic. The most perfect example of this in the modern era is of course the American Republic, which is a powerful oligarchy controlled by a few select institutions and families and their patronage networks within the beaurocracy of the State.
The difference between a Monarchy and Republic however is that the pretense is dropped entirely. In that respect its more honest. More importantly however, Monarchy divests the ruling elites of their implied legal authority given to them by the people. Instead the contract between a Monarchical Aristocracy is constantly being renewed and earned, whereas a Republican Aristocracy can dispense with any notions of giving shit about the common plebes because they are just voting cattle who will do as they are told.
The idea that because your continent spanning Republic you no longer have to worry about corrupt nobles is a fallacy. The nobles are the same, they just have different titles. Instead of Lords and Dukes, they are CEOs and Secretaries.