Generally people are lazy & it’s a case of ‘out of sight, out of mind’. When I leave a job, it’s very seldom that the people left behind at the office make an effort to keep in touch. They’d rather maintain friendships with the people they see every day at work. Plus if they’re married with kids, they’ve prioritised those relationships so yet another reason why you don’t hear from them. Also, many friendships aren’t built to last. Circumstances & people change & the reason many friendships fizzle out, is that people find they no longer have anything in common anymore.