[djur]

GitHub doesn't allow private forks of public repositories. If you read the licensing terms, you aren't allowed to distribute modifications of the UE4 source to anyone who isn't already licensed. In addition, a lot of users of UE4 would not want to make their changes public.

Also, they would be required to allow GitHub users who weren't UE4 subscribers to create forks, which is a violation of their licensing terms.

Public GitHub repos just aren't really suited for projects that aren't open source.