In a society with a harem based reproductive strategy competition between males is exacerbated and the top 20% of men wield most of the reproductive power while the dysgenics are rightfully disenfranchised. Thus, if anyone is going to be the most successful shouldn't it be these types of societies?

In reality every society/tribe where it is/was the norm for men to have multiple wives is low iq and dysfunctional. It is easy to see why tribes that embraced non-monogamous reproductive strategies like polyamory or cuckoldry proved to be inferior but I can't figure out why that is the case for harem based societies. When looking at any great nation/society in the world it almost seems like monogamy is a prerequisite for it to be successful. Why does monogamy MOG?? please discuss thoughts.