Eh, genes exist. Family culture exists. When you measure traditional aristocracies, such as Indian Bramin and European Nobles, they actually do outperform the general population on a variety of stats. Because they really do have better genes as long as they don't fuck it up with excessive internal breeding. Now, partially that's because these groups tend to be a bit more meritocratic than commonly understood (that brave knight might have been promoted to that position, or had started out as merely a knight, then gotten promoted to a Barron through good service).
But, if you have a class of society that promotes into itself the top, say, 0.1% of the population outside of it into it, and they do that for a 1,000 years, basic evolution suggest your going to have a class made out of authentically superior people. It actually makes perfect sense that the top leadership should come from some tiny slice of the population in a functional, established ruling class because a functional ruling class would have already taken the top 1% of families into itself, so new entrants only really come up as the product of random positive mutations.
Why we don't have quite as established a ruling class (though we have quite a narrow range of families in charge if you look into it) is that what makes someone "1%" has greatly varied and changed in the last 200 years: The fuedal order could retain a bit of stability for so long probably in part because what made a good noble (material skill) didn't all that radically change for about a 1,000 years.
The changing nature of warfare and the economy however changed what it took to be an elite, which created a disconnect between who was in power (decendents of aristocratic warrior castes) and who was the actual source of power (the "capitalists" for lack of a better word). Thus the various revolutions. Excessive interbreeding, especially at the very top, and the nobility getting less meritocratic (partially because they became less important: see France where the nobility went from the source of the Kings power in a knighthood that performed very well (outside some obvious hickups against the English) to something the Kind purposefully tried to sideline as an inconvenience) and thus less effective of course didn't help.
