I would not be surprised if the absence of collapsible threads were a conscious design decision. To put it another way if returning to the same thread again and again was considered a user behavior to be slightly discouraged, then your anecdote indicates why no collapsible threads could be a feature rather than a deficit.
Thinking about why a designer would wish to raise the cost of returning to the same thread again and again: the cost is still rounding error when the reason is a compelling and collegial exchange of ideas and relatively high in relation to other places on the internet where people type into boxes when the motivation is pouring gasoline on the embers of insults, bullying or trolling. Or just ordinary XKCD386.
Another habit it discourages is a clique of people who hang out in a few threads for prolonged periods (say a week or more) engaging in 'insider' tribalism. Instead it may encourage people back to the front page and the content. On Hacker News, interesting topics recur when people make new submissions of additional information instead of prolonged based on the same information and position defending.
None of which is to say that this is in fact fact or to in any way imply that these habits or motivations or behaviors apply to you individually. I think it's more of design to avoid tragedies of commons.
At the very least, I would suspect that particular bit of negative reinforcement failed at its job. A number of other features (karma primarily, but also the comments page and saved stories and saved comments) seem to encourage long term engagement far more than non-folding threads could discourage it. That downvoted comments alone appear on /newcomments practically makes it a flamebait generator. Tribalism, trolling, vindictive nastiness and even meme posting abound here.
On the other hand, I suspect (with nothing to back it up) that pg simply never considered thread folding, or else didn't consider it interesting. Meanwhile, any new feature will inevitably garner criticism from users if it doesn't work they way they expect, doesn't support their browser, etc. And, anything that causes Hacker News to look and behave superficially more like Reddit will cause more than a few users to rend their garments, put on sackcloth and ashes and wail in agony at the death of Hacker News.
That headache, combined with the 'unique' (and not really open) nature of the codebase, a staff with better things to do most of the time, and a strong cultural aversion to making even small changes in the UI, means otherwise apparently simple changes probably take far more time to roll out than many developers would consider reasonable.
For what it's worth, if indeed PG didn't consider thread folding interesting, I cannot say that I blame him since it is that way for me. The feeling that scrolling through a page to find some comment feels like a bother means that it actually is a bother and not worthy of the effort. Collapsible threads making it less effort to rebut won't increase the quality of my comments...on average, it will probably reduce them. YMMV.