>I've never understood why HN karma can't be a proxy for the kind of behavior one should expect on lobsters.

Because karma doesn't actually indicate what it's meant to. People don't just upvote comments for their intellectual quality and civility, nor do they downvote comments just for their rudeness and banality. All karma really measures is interaction over time, and high karma if anything is a sign that someone has nothing more productive to do than hang out here.