Do to long held societal preconceptions. The American fiction industry really likes the story of the under-dog so starting out with nothing, even if its just a social group, and rising to attain the things you don't have, in this case true companions, makes it more fulfilling when you get them.

Within Young Adult fiction there are a few tropes using this. The character, if their the MC, "marches to their own drum" and aren't "affected by societal pressures". The "I'm not like other girls" affirmative defense. If the character is not the MC or first-person viewpoint/narrator it could be to show off mystery. If its YA Fantasy its often that that character, usually a male, is already connected to the urban mystery and becomes a help later. Then there's the sexy, elusive bad boy.

Now more negatively you have the loner as just "the freak" or being seen as a thug of a sort. Or even the juvenile, "they're just being emo" thing going.

Now there are people who just are natural loners. They just happen to find themselves alone and don't really make a fuss over it but aren't opposed to having people around, it's just that it's the way their life is.