It's easy to re-open a file (assuming it's still there), but with sockets your IP may have changed, the remote IP may have changed (which you may have stored in your working memory that got checkpointed), DNS may point you to a different service entirely, you could have had to do some kind of port knocking or something to get that connection open in the first place.
I know this kind of stuff is being worked on so VMs/containers/namespaces can be moved around but it seems to be one of those things that gets really complicated when you try to do it transparently for userspace.
IPs, DNS settings change on running programs all the time, that doesn't seem as unusual as re-opening a file that's actually not there. A unix socket is an interesting mixed case :)
I know this kind of stuff is being worked on so VMs/containers/namespaces can be moved around but it seems to be one of those things that gets really complicated when you try to do it transparently for userspace.