Because git is the latest fashion rage and by itself incapable of managing the size of the Windows code base. This is the ideal combination of reasons for corporate PHB to decide to use it.

Exhibit A: GVFS was solely invented as a hack to make git usable by the Windows team according to the Windows team blog.

(Yes, there is some sour grapes and sarcasm in this post)

Does GVFS support fine-grained access control to parts of the repo, or all userss are guaranteed to access the whole repository if they want?