

But if women have a biological imperative to mate with assertive, dominant men, then why isn't the population of men already made up of 100% assertive, dominant men? You'd expect the submissive "betas" to have been selected out of the population by now.
Not exactly. While someone with "bad genes" usually produces children with "bad genes" someone who has "good genes" doesn't always produce children with "good genes". Mutations happen and most of them are bad.
Being able to walk upright on two feet is key to reproductive success, and thus the fraction of babies that are born crippled (due to mutations) is quite low. If being dominant and assertive has been key to reproductive success among men for a long time, one would expect the fraction of male babies that are born nondominant/nonassertive (due to mutations) to be quite low.