Why doesn't git-rerere automatically commit a resolution?

I'm curious... I have a pair of topic branches which don't merge together cleanly by recursive (due to conflicting hunks in the same line segments). I enabled git-rerere, ran the merge, fixed up the hunks and committed it. git-rerere built its cache, as the next time I pulled the two topic branches together and got the same conflicts it correctly regenerated the prior resolution (and printed a message saying as much).

But it git-rerere left the files unmerged in the index and it doesn't generate a commit for the merge, even though there are no merge conflicts remaining. I expected it to update the index (to merge the stages) and to generate a commit if possible; especially in this case as I was pulling the exact same two commits together again with the exact same merge base commit.

So I'm wondering why doesn't it try to finish the merge? Was there a really deep rooted reason behind it or was it simply easier/safer to let the user sort out the working directory state every time?