Some girls do it because they are willing to and/or have a sexual attraction to other girls, most guys don't because they are not willing to and/or do not have a sexual attraction to other men.

Studies show that females are more likely to be attracted to both sexes than males are. I thought this was fairly common knowledge. Guys are more likely to pick one gender and stick with it.

As for "is it common" - I would say yes and no. It's common in some cultures, it's less common in others. It's common in some circles of friends, it's less common in others. I've seen it happen a few times and I know a number of girls who have made out with other girls. Gotta say, it's a lovely thing to witness, for whatever strange reason.