This is what traditional forums excel at - for instance, a niche-specific forum may also contain off-topic boards for topics unrelated to the niche, such as general current affairs or media.

wrt to Reddit, I mean, Lemmy does exist, but much like Mastodon instances try to be "another Twitter", Lemmy (instances) also tries to be "another Reddit". At the end of the day, it's a human/userbase problem of "platforms change, hivemind doesn't".