Here's the answer Jeff's referring to on why this doesn't happen currently:
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We may revisit hooking up comments in the future, but it's not a completely trivial problem (e.g. should reply to hook up as well? that's more info that needs carrying over), and won't/can't be retroactive. That being said, I do have an idea on how to take care of this in a not terribly expensive way, I'll throw it out to the team and see if it's worth it.