This has been the case for me throughout my life, mainly people I was friends with at various points in school. Friends in elementary, stopped seeing each other once we went to different schools. Had a friend throughout school, and after graduation, we've not seen each other since.
Though I did have a friend outside of high school for a few years (the guy that introduced me to Needless and anime that wasn't surface level). We had regular meet ups with him and his other friends, but since moving to Redmond to go to DigiPen, I haven't been in contact with him since. Weird thing is we're both friends on PSN, but he's like me in that he doesn't typically go out of his way to maintain contact, so it's both on us we haven't said anything.
Basically, how I deal with it is this: don't give a shit about it. Be content with having acquaintances throughout parts of your life and move on. Of course, that's only really applicable if you're like me and have no problems with being alone in the first place.