For me it's interesting -- I'm somebody who has in the past made the niceties when somebody reached about but ultimately not reciprocated much. And it wasn't because I didn't care, it was because never in my life had somebody reaching out from the past resulted in genuinely satisfying interactions. So in a certain fatalistic way, I had decided it wouldn't amount to anything which was then possibly self-fulfilling.

I used to believe that whenever coworkers ask how to stay in touch after I leave a job it was just a lie we all tell ourselves to make the transition easier. But I'm finding that many of my coworkers say they do stay in touch with old colleagues.

Perhaps part of the lesson is that some of us who don't reciprocate with you do care, but have no mental model (or lack the social grace) to navigate a more tricky relationship (e.g. what if work is now much better now that this person left, and what if the people and things you had in common no longer apply).