culturally, why might females need to meet higher standards of attractiveness to be considered as credible as males?

It may have to do with more olden stereotypes, where women were seen to be caretakers of the family, while the male went out to work and earn for them. Many jobs back then were dependent on strength, so as long as men fulfilled their role of earning money (through labor) their looks were seen as little importance. Let me elaborate. If a man could care for his family, he was seen as admirable, or at least acceptable to society. If he couldn't earn, then he was deemed worthless. So a man's honor depended on how his strength was. In this sense, looks didn't really contribute to a man's place in society. Women, on the other hand, were the face of the family. Men would show them off to friends, so if women weren't decently attractive, then it wouldn't be very good for the male. Other than housework, the woman's job is to literally sit still and look pretty; thus the reason behind corsets, bonnets, petticoats and so on. You won't see society asking men to wear such complicated things, because their main job is to work. It would be unsuitable for a blacksmith to be wearing tight suits while hammering, no? I hope this gave you some general insight.