Edit: Youtube would never do that, because reading comments distracts from watching videos which reduces ad revenue.
My guess is that it's just impossible to regulate. Take a website that has state of the art threading, karma system and all that: reddit.
AskReddit threads sometimes get to 10k comments, it's just impossible to read through (the bottom 5k comments will never get read, much less upvoted). And people on Reddit sometimes say that their inbox "blew up" and what not when they get a 100 responses (which is a lot really).
Can you imagine youtube-level traffic? Imagine a video with 1M views (which is really not a crazy number by youtube standards): if 1% of users dislike your comment enough to reply to it, you'll get 10k responses! Even if we cut this number by 10 (assuming that 9/10 users either don't bother reading or replying to your comment), you'd still get 1k comments. Even a tenth of that feels like a large amount.
Now, also imagine people being able to read through your comment history and go on witch-hunts like people on Reddit sometimes do (and which every moderator tries to prevent), but on youtube audience scales. Oh god…
Just like with any old primitive forum.