I agree with fast scrolling as the cause. All our social media these days emphasize endless new content to the point where it seems almost nobody reads the actual article anymore. I've seen posts on some of the politics subs on Lemmy where it's obvious not a single commenter actually read beyond the headline because they're totally missing some major point.

As to why they feel the need? I don't think it goes beyond validation. People know the sort of one-line comment that will get them a handful of upvotes and agreeing replies, so they rush to be the one to make the joke first. It really becomes a drag after a while when what you're looking for is actual discussion of the article. I find myself spending more time on Tildes than Lemmy because those sorts of low-effort replies are discouraged there.