To me, this is a novel insight, thanks :) I have been wondering what has been causing my frustration towards these kind of subreddits. But this makes it clear that the problem is a lack of actual discussion and no challenges made against the assumptions of the in-group.
I have noticed this is a broad trend on a lot of the internet; the internet has a lot of data, but most of that data is repeated a lot. If you want new and interesting information you often have to look elsewhere (e.g. books).
A particularly annoying example to me is the ausfinance subreddit which is obsessed with a housing market crash that they seem to think will definitely happen tomorrow. Anything pointing in that direction immediately gets extrapolated as the horseman of the Australian housing market apocalypse, nevermind the fact it was trending the opposite direction yesterday.