GitHub only shares objects among forks. Source: I used to work on GitHub's Git Infrastructure team, but this is publicly available information.

You can read about their architecture in their discussions of Spokes, which replicates repository networks (the original repository and its forks) across data centers. eg: https://githubengineering.com/stretching-spokes/

Trying to put all of GitHub's object files in a single packfile - even just putting them on a single server - would be impossible.

But even on hosting providers with a bespoke implementation - that do not use core git to manage Git repositories - this would be challenging. We have a custom Git server implementation in Visual Studio Team Services, but it still makes sense to shard object storage with the repository: you have to worry about scalability and performance, but also things like data sovereignty. We can't just put a user's git repository in some global SQL Azure database that contains all the repositories in Visual Studio Team Services: the repositories need to be geographically located with the VSTS account they created.