Well, a lot of friendships are relationships of convenience. You are friends because you share some qualities or interests and happened to be in the same place at the same time. If the only real connection is based on proximity, then it shouldn't be surprising that the relationship falls apart when you lose that connection.
My two closest friends I've met in middle school (like 14-16 years ago) and I still see them regularly, still have contact with an internet friend I met around the same time playing RVS, and I'm probably meeting a friend before christmas I met 16 years ago, my oldest acquaintance with whom I still play/talk and visit, although with that one we've lost contact for a while. Still have contact with college friends as well.
Those protagonists in manga have no excuses! That said, I'm not secretly in love with any of them, I guess that helps reduce the angst.