It's really interesting. Reading the comments look like the primary reason to not maintaining friendship is mobility. I always suspected that humans are not well adjusted to that, think about what was a terrible punish back in time, exile.

Exile meant loosing your friends and family network and I think we experience a mild form on this in current times (not nearly as bad)

I'd never made strong bounds at work, my friends are the same as I grew up but I'm still living in my hometown (except for brief periods of working outside) and a lot of them are also here, so it's easy to maintain that friendship.