That's cool. I wonder why Git hasn't copied the implementation.

They are completely different. In fossil, commit history is sacred, where git users can rebase, and cherry pick to their hearts content

I meant the implementation of these side-features as objects sitting in the repo, with appropriate client commands for creating and editing them, etc.

Git already has notes, and signed commits/signed tags, which are all those same kind of "objects that just happen to be there." So they don't need to copy Fossil's architecture; they can just copy the way that said object types interact as dependents of commits (while letting them get blown away when commits themselves do.)