IMHO different purposes. From highest to lowest content value:
Forums: Topic-specific, ongoing discussion, like sitting around a table with a sign above it saying what it is about (title) and people discussing one at a time, giving everyone time to think about their arguments and speeches. Slow, effective, tends to get off on the wrong tangent, and therefore prone to bashing until someone reminds everyone what the title was. This slowness is what makes people think, and therefore prioritises the accumulation of knowledge. I've seen plenty of forum threads where there was no answer at first, but slowly a proper one emerged. The big disadvantage of forums is that, as an outsider, you have to find the answer deep in the threads. There are now mechanisms to promote the best answer, chosen by the poster or by voting. This helps, but you get the same side effects as with message boards. Speaking of which...
Message boards (Reddit, Lemmy, 4chan (technically), Mastodon, Twitter,...): Focused on a discussion, but less on textual content, more on media content. Where forums are all about speeches and discussion, message boards are all about quick comments. Great for having fun, not great for getting people to think about what they are saying. Adding voting mechanisms simply solves the problem of searching for the best answer, but the nature of message boards makes it more rare for people to create that best answer. High thread throughput mitigates this to some extent, but it's exhaustive on all parts (infrastructure, mods, participants). As a side effect, voting encourages radical behaviour for the sake of higher self-esteem, stifling niche discussions and encouraging broad topic superficial discussions. I have to add, though, that doing this karma stuff like on reddit, where you accumulate upvotes in a giant imaginary bucket, has worsened these side effects enormously. Voting is not a bad idea if it's done as simply as possible, so that it doesn't lead to posting just to fill an imaginary bucket with imaginary points.
Chats and chat-focused apps (IRC, Discord, Whatsapp, ...) focus on small, quick messages. Long discussions are rare, as no one except the active participants at the time of the discussion benefits from finding a good answer to a question. Especially if looking it up means going through a history of short messages one by one to understand the answer. That's just not the point. Chats are literally just that: A quick conversation between people on the go. You're not going to write a proper master's thesis by chatting, although it might help you find your way from time to time.
And finally, the comment sections. This place was never intended to be a place of knowledge creation. It's just a big open space in front of a stand where people can randomly shout stuff at random people about what they've seen at that stand, usually with no intention of getting better opinions. It's just "here's my opinion, do what you want with it".
As Mark Twain once said: "I didn't have time to write a short letter, so I wrote a long one instead".
People started putting all kinds of fillings into sushi like chicken, beef, sausages, vegetables and whatnot. So... fresh fish ain't a requisite for sushi anymore.