Why do so many international students form a clique?

People tend to hang around those they are most comfortable with. I'm American and when I studied abroad, most the Americans hung out together. It's just a natural human instinct. It's also possible that they don't feel welcomed or understood by the rest of the student body. My suspicion is that if you keep reaching out to them, they will realize that you are sincere and they will be interested in getting to know you better. Just try to put yourself in their shoes and don't take it personally.