They don’t have to be less fortunate, other countries have wealthy individuals as well though less common. So a man of lower relative value due to dating someone in the US would be able to get a more valuable genetic partner due to the improve relative value of having us citizenship. For example a construction worker or male nurse in the US could marry up by Marrying a Columbian doctor.
they say they want traditional values, but honestly they want malleable women. there are many conservative women in their own countries but it’s much easier to control someone who is dependent on them, may not speak the language of the passport bro’s country, and might not have any means to get back home without the passport bro.