Specifically the thing that Git was meant for was there being multiple authoritative roots. In the sense that RedHat can keep their own kernel line and just bring in patches from Linus and other maintainers as well. Everyone can be authoritative! This is great for example if you need to fork a library to make some changes (eg replace a library it's using in it's implementation) but want to keep current with it's improvements.

...this is not the case for most projects. They want to have a clear source of truth for the code.