Well, the problem is that polygamy is not good for society, even though it is good for some individuals. You end up with a bunch of rich older men with multipl

Well, the problem is that polygamy is not good for society, even though it is good for some individuals. You end up with a bunch of rich older men with multiple wives, and very limited romantic prospects for younger men. And you definitely don't want large numbers of sexually frustrated young men in your society. We've been moving in that direction (toward de-facto polygamy) for 50 some years now, with the pill, breakdown of traditional marriage, societal acceptance of premarital sex, etc. Luckily, entertainment outlets (porn, video games, etc) have also improved dramatically, providing an outlet for some, but I don't think that is necessarily to the benefit of society either.