I have to protest here.
You make it sound as if monarchies were inherently wrong/evil. They're not.
As a matter of fact a "good" king (in both the meanings of "moral" good and "capable" good) can likely do much more than a "good" president.
The problem is that once you have the misfortune of getting a "bad" king it's much harder to remove him from power than it would be for a "bad" president,with all the consequences that follow.
Mlp Equestria had the advantage of getting not only GOOD princesses, but also immortal ones. And even then Luna went bad, proving how the system was unstable. If Celestia ever died it would have been chaos. They were lucky they had plot armor until Twilight appeared.
As long as we get good heirs the system will work. Once we get bad ones... Well, we have the advantage of the near-immortal regent her majesty best dragon daughter. That will likely be enough to let the system survive a couple of bad apples.
It is certainly true that monarchies are not inherently evil any more than any other form of government. They simply don't incentivize those in charge to do things which are beneficial to the people.
While a king who happens to be morally good could potentially help his people a great deal, it's incredibly difficult for him to do so. A king must keep his key supporters happy, just as much as a president must keep the people who vote for him happy. The difference is that there are fewer people that keep the king in power.
On the surface, it may seem as if it should be easy to keep the king's key supporters happy, and then get on with the process of wisely ruling the country. Have a kid, go to church, make sure to give enough bribes to the nobles. However, in practice keeping these supporters happy is a difficult and expensive process. There are many examples of kings or other leaders with few supporters (who I'm going to call kings for the rest of this post because it's faster to write) who have indeed tried to make things better for the people. Most of them promptly got stabbed to death or shot, because they misjudged how much money they needed to use in order to keep their key supporters happy. It takes a lot of money to rule wisely, but it also takes a lot of money to keep your key supporters happy. That food you want to send to your starving farmers? Why, that money could instead be used to pay for a feast for your nobles. Letting the serfs move freely between nobles estates so they can find a better life for themselves under less oppressive nobles? Duke So And So is not amused by your attempt to let his property run away.
A good example is Julius Caesar. He was well loved by the common people of Rome during his time as a dictator, in part because he started many projects to help them such as restructuring debt to reduce the number of debtors, imposing term limits on governors, and creating public works projects to help reduce unemployment. He was soon killed by his key supporters, who were very much
not the general public and did not approve of how some of these policies affected them.
Yes, it is possible to get a good king who helps the people. It is much more likely however, to get a good (morally speaking) king who makes life even worse for the people in order to stay in power, or a bad king who makes things worse for the people in order to stay in power and live in luxury. These last two categories are very difficult to distinguish from each other, so I'm grouping them together. A good example of a 'king' who has indeed made life better for the people is the leader of China, showing that it can indeed happen. The examples of kings who make life worse are much, much more numerous.
The problem isn't that humanity keeps getting kings who are terrible people, nor that we keep getting kings who are incompetent. The problem is that in order for a king to help the people he needs to be both a very good person
and incredibly competent. He must be morally good enough to go out of his way to help people and to also potentially accept a lower personal quality of life for himself, as any money he spends on the people will be money he would otherwise spend on himself. He must be extremely competent because he needs to not only find policies that actually help his people, but also to make sure those policies don't anger his key supporters too much. Finding policies that will help the people is of course only made more difficult by the lack of free press that is necessary to keep the people themselves from rebelling and overthrowing him. He also must do the delicate balancing act of finding money for these projects while making sure to still have enough money to pay his key supporters.
This is the reason that there are so few real life kings that helped the people. It's not that kings, queens and dictators are all inherently bad or stupid people. At a base level, they aren't much different from anyone else. The reason is that they were never incentivized to help the people, and it takes a truly exceptional person to both make the effort and actually succeed without getting killed.
Contrast this with politicians in democracies. Are politicians somehow naturally morally superior to your average king or queen? Of course not. However, they are in a very different position. Politicians are incentivized to do things that are good for the people, or at least which the people want. This is because the people are key supporters of politicians. Keeping the people happy directly contributes to a politician staying in power. It's a system that has many, many flaws and many failures. But on average it works a lot better than monarchy or any other form of government that relies on a small number of key supporters.