Probably because most interactions were on company time. Because of course if the relationships were outside of work, then changing jobs would have little effect.

If those ex-colleagues were truly good friends, they ought to find the time to hang out with you outside of work.

This would likely lead to genuine, more interesting conversations.

In my experience, true relationships survive (and sometimes thrive in) a departure.

Relationships don't need to be "true" to be useful.