Or they could link statically. They're both under the LGPL, and it seems like the sensible thing to do if they're only using the library for hash tables...
The project authors certainly don't care about people with 'special' requirements. For them it just saves time.
Now the packagers _could_ do what you suggest, but especially for such a central library it doesn't make sense. You want to have only one version of that thing for a gazillion reasons.
The original request for this thread? Well, it seems that person wanted to port a library to a completely different system _and created the binary for that system_. Right, he can statically link it. What does he gain? He still needs to gather all dependencies before and his complaint (as a developer/distributor) of having to meddle with glib would be the same. He didn't complain about distributing another couple .so files as far as I understand the issue.