probably because even the most assholish of assholes still have relationships and get married. it isn't something exclusive to nice people.

and in a society that values looks above personality, it can be confusing to see people of, how do you say this politely, 'lesser' looks getting into relationships when others act like a repellent to anybody of the opposite gender.

i know from experience. i like to think i'm a fairly nice person, but many guys look at me as nothing more than a friend because i'm not what you would call 'pretty'. i can see it in their eyes as soon as they meet me. there's no sexual/romantic interest there. they take one look and think 'well, she ain't hot. tick her off the interest list'.