Why are girls so mean to each other?

So I'm a 21 year old girl, with two older brothers, and have very few close girlfriends that normally hang around guys like I do. When I was younger, I suppose I was just more comfortable with guys because I had two older brothers. As I got older though, and became more in touch with my femininity I wanted to have a bigger group of girlfriends, and have girl nights! I found however that when a group of girls get close to each other, they're extraordinarily not nice, when looking deeper than surface level, though when first getting to know them they seem like sweethearts. They gossip about their close friends, and put down girls they find less pretty than them, and talk about how they're jealous about prettier girls when they are not there. In addition, the ones that have boyfriends spend so much time talking about their significant other. They're also extremely cliqu�y, judegemental and close-minded - and I feel like they try to maintain and conform to chauvinistic and traditional conventions of women. It really bothers me, being a young woman, as I find guys so much more honest, open-minded and easier to talk to. I feel like I can be myself with my close guy friends and those few girls that don't follow social convention of how women should behave and not be judged or criticised or talked badly behind my back. I know not all women are like that - as I have few close girl friends that are not. I just really don't understand where women are coming from when they complain about objectification and ism, about men, and tend to behave in ways that attack one another. I also find the loyalty value is missing for women. I have a close guy friend I've known all my life and when I introduced her to a new girl friend I recently met, she tried to subtly cut me out and get closer to him. I find that happens a lot too, in which girls try to put each other down or cut each other out just to get closer to a guy. It makes me sick. P.S. I'm entirely straight, and my friends all consider me extremely feminine, which is a big reason why I'm so confused. Can someone clarify the mindset of women (in general) for me?