It happens to boys too, although not as often. One of mine got completely ousted from his friend group at the end of middle school. They went from all being excited about going to HS together to not talking to him or including him. He claims he doesn’t know what triggered this. It did get better and he want on to make very good friends, but it was hard. I know other boys that got “dumped” from the “cool group” that kept going back and trying to be friends with those kids, but got nowhere.
I have seen kids in the "nice group (for example band kids) be incredibly exclusive, sexist and just plain mean to their peers. The teachers and parents were still clinging to the idea that these were the nice kids and that the higher status athletes were not, which was not the case in this situation.
It is not always the parents. By late elementary or even middle school, parents can’t really control how kids treat each other when the parents are not around. One parent I know was horrified that her daughter was one of the mean girls in 5th grade. She constantly talked to her about treating people right, but couldn’t force her to stop being friends with the kids she wanted to be friends with or to befriend girls she didn’t like. Many kids are drawn to the popular group.
Also, your perspective can depend on context. The view that a group of kids are “really nice” is typical when a kid is in the “in group”. The view that the same group is cliquey and mean, is just as valid from the kid that is not in the group. Is not being friends with someone necessarily being mean? It may feel that way (and we all have probably been on both sides - there are people who don’t like us for whatever reason and people that we don’t like).