Why should the smartest people rule when you can have the most brutal? Keeps the population manageable and keeps them in line.
I mean, it's a strawman argument (this is Shitpost Central after all), but people tend to not like being controlled by others. So what's the point of society if it doesn't make people happy? (to answer my own question, perhaps an authoritarian society can do a better job at making more people happy than a non-authoritarian one.)