I feel like a closer approximation of what’s going on is that there are perennial discussion
topics that people want to participate in when they see an opportunity (e.g. “battery technology continues to improve, but it’s rare that we see the results trickling down to consumer electronics.”)
But just having a forum with “evergreen threads” where people talk continuously about perennial topics, doesn’t seem to work out very well, as people tend to get bored of topics when nobody’s saying anything new.
People might want to participate in them, but they won’t bother if there’s no expectation that their comment will be seen, let alone replied to. Nobody wants to post to a “stale” thread; it’s a waste of effort. And, in a vicious cycle, nobody wants to read other people’s posts on a “stale” thread either, because “thread necromancy” is almost never done by someone with something of value to contribute.
“Post a link and comment about it” sites have stumbled upon a fix for the problem of perennial discussion topics becoming "stale": by having individual threads focused on news articles relevant to these perennial topics, rallying points are created, giving people an excuse/justification to resume the larger ongoing topic of conversation within the comments of the individual news item, with the expectation that others will also be doing so.
The news itself is important to this process, as it offers a promise that fresh content (i.e. comments with a novel thought backing them) might be injected into the ongoing topic due to the news; thereby giving people a reason to check out this particular resurrection of the topic, even if they’ve read previous threads “about” it before.
But it really only has to be a promise that someone else is going to contribute novel thought to the conversation. Most people don’t want to be the source of the novel thought themselves. They want to participate on one of the existing sides of the topic, using what they already knew about the topic going in. They want to try out a “famous” debate for themselves. (And, if there's something new to know, they seemingly want to learn it by being told off by someone who has read the news, and so now knows something they don't.)
So the news article itself has about the same effect on the volume of commenting on a site like this, as public news regarding a company’s value has on the volume of trading in a market. A dose of “the real thing” certainly causes a surge—but so does the expectation of it, whether you actually end up getting the real thing or not.
And, just like speculating on a stock doesn’t require any insider knowledge, commenting under one of the “traditional” positions on such a perennial topic doesn’t require that you read the news that prompted the resurrection.
You can't have a stock market without real signal; and yet the market will always be composed mostly of traders with no access to (or desire to access) real signal. You can't have a perennial discussion without news; and yet the comments will always be composed mostly of people who have no desire to access the news.