The source for libgit2 itself is not present on their opensource site, though they use it in Xcode. (Git itself is present, but is sometimes out of date compared to what they're actually shipping.)
I've emailed them about this and they offered to provide me the source they used. This feels like they're trying to do the bare minimum of compliance - and making even that nonobvious (since they have an obvious distribution mechanism.)
This complaint is not a claim that they're violating our license, but it's definitely not an ideal situation, and I'm sympathetic to your frustration.
> In addition to the permissions in the GNU General Public License, the authors give you unlimited permission to link the compiled version of this library into combinations with other programs, and to distribute those combinations without any restriction coming from the use of this file. [...]
It allows people to link it in their non-free, proprietary software without any vitality on that proprietary software. It still requires them to make the source for the version of libgit2 that they are using available upon request.
No, the reason they don't put it on GitHub is that they have no intention of participating in the community that made the software they are using.
All of these tools already have sites, communities, repositiories, etc. Apple could easily participate as many other companies do who also have proprietary patches. Apple actively chooses this over being a member of the software community it takes so much from.