As if simply being born to power or related to it makes one a great ruler
Well, assume you took the son of a brave, clever Knight who won many battles and had many successes, and trained his son in his image...
And then took the son of the town drunk, who trained his son in his image ...
Which one is more likely to be an effective leader? Or, take as given the normal distribution of traits on a spectrum, like IQ, as a given:
Now, if you had nothing but lineage to go off of, is the drunkards son most likely to be in that top 2%, or the Knights son? Even ignoring biological element of good breeding, just the cultural elements of good breeding. Now, part of why we invented things like the IQ test is so that we have something besides someone's lineage. But, a good leader is a wide variety of things, not just one easily tested and quantified thing like IQ. Eventually, you have to use very costly measurement attempts, or use very approximate rules of thumbs, like lineage.
Even if its a generally poor measure, its still something of an effective measurement: If someone's father is in the top 1% of leaders, even if the son's less impressive than his father, he's still probably in the top 10% of innate capability. Meanwhile, a random selection of the median is probably less likely to be in the top 10% than the son of someone in the 1%.