You act like this type of mentoring isn’t being done. It is. I work for a Fortune 200 company in a city having several Fortune 500 companies and they all do this.
I only see the start of the pipeline. You're right though, it'd be nice to see it all the way through in order to better understand what overall impact we're having, if any.
Additionally I think we all can do grassroots mentoring. I know the challenge is discovering a good candidate, but it's an option. Buy someone a simple laptop, and teach them from first principles. (I've seen at least person I know and respect do this on Twitter)