Because those friends will end up going to university or moving on with their lives in another way. Only a few will stay. I speak for myself, I only keep regular contact with 5 or 6 friends from highschool. Making new friendships, meeting new people is always exciting and university is the best place for it. Not to mention university it's full of girls to have sex with and weekly parties to go to. Discos are horrible to have fun. Let's just say university parties are much better, specially the ones in the campus where you can get some privacy with the girls.
More importantly, you get out being a new person, a new better version of yourself because you end up experiencing many new things.