I think part of that is because these libraries do not live on github, or have different identities attached to them (different email addresses like @gnome.org

I think part of that is because these libraries do not live on github, or have different identities attached to them (different email addresses like @gnome.org for libxml2 stuff). It is essentially a hard problem, one that ohloh.net (now openhub) tried to make fun a while ago, to encourage people to claim their own contributions to tie their identities together. I do wonder if they could have used openhub data to improve identity links though. Still doesn't solve the dependency graph lacking.