Well, random sorting might be a bit of an annoyance due to HN's other annoyance, the "Unknown or expired link" problem, which seems to crop up more often if I do actual work and then pop over to HN when I have to wait for tests/compilation/etc. Locating the comment you were replying to with random or date-based sorting (so that you can click "reply" again, paste your previous reply, and then resubmit) would be a serious pain in combination with that.

But you're correct in that the current algorithm (or any sufficiently transparent algorithm) does lend itself to gaming. It might be harder to game if you didn't mind penalizing comments based on the quality[1] of replies they generate, then sorting based on the thread's average quality would mean that there's less incentive to post in the top thread just to get exposure, since your new, karma-less post will drag down its average and subsequently could drag down the thread. That could get prohibitively expensive to calculate for huge threads, but would be less prone to gaming, I think.

[1] If we can hand-wave "karma" to mean "quality"; I get way more upvotes when I am flippant but technically correct than I do when I try to be thoughtful.