I still really don't get the 'Graduation might as well means the other person died' in Japan.
At my graduation, I was like 'Well... tomorrow at your place as usual?' and next to nothing changed.
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. Presumably these two are going to different schools after graduation. Kids can change quite a bit in a short period of time, so keeping friendships with people you don't see every day can be challenging.