This isn't really the case! And, FWIW, it is neither true that monogamy is some clever invention nor that many women are meaningfully hypergamous. In all the high-quality data I see, women are just single more just as men are, they just don't talk about it as much in places you or I read (it seems like psychometrics implies the median woman is more okay with being single than the median man. Part of it could be that).