Beside from MC being clearly blind, like not realizing the gender of classmates or the cousin he is living with, story doesnt give an actual reason for why dont other girls approach him either.
I think author is going for something between "girls think they have no chance/ he is not into them" or "they think he is with Rui" but both of them are ridicilous and unrealistic. Like, polygamy should be accepted for that world to function in the first place. And both the girls in his school and random women in the streets would have asked for his number / dates regardless of his behaviour.
1:5 manga and 1:32? (not39) are way better than this in realism, with male mc getting approached by random women everyday. And 39 is at least aware that its satire, so this leaves 1:20 as the worst of both worlds.