I typically don’t call my friends girlfriends, but I’d imagine the reasoning is similar to what @josie gave.
When someone refers to another as their girlfriend, it implies they are very close. Probably because that is a term most boys use to refer to their significant other. I see girls that are best friends walk arm in arm/ hand in hand in the hallways at my school. They are close enough to do that. I never see boys do that. Most likely because they don’t want others thinking they are gay. Gay teenage boys get a lot of shit from other boys, from my experience. Probably because it’s seen as “Unmanly”
@avoice I don’t think that “Clingy clique behavior” is annoying. Maybe it’s because I get like that sometimes with my friends, but I think it’s just natural. The giggling and stuff is annoying, but that’s about it.
As a semi-related side note, it’s apparently becoming “offensive” for a straight woman to call another straight woman her girlfriend because it confuses LGBT people or something along those lines.