The huge number of communities that have elected to move to Discord, an information black hole, seems like a strong counterargument to the generality of this claim.
Was a bit late for IRC's heyday but am I right in thinking it wasn't guaranteed that all users would have access to the chat history? That would create an impetus to log the information elsewhere.
Private trackers such as what.cd tended to attract people with strong archivist streaks which I think largely resolved the issue there. Oink could go down and you'd know that a huge chunk of its catalogue would appear on Waffles within a few months.
Online marketing feels like a field that would _need_ a private place to talk tbh; anything public would be a beacon for self-promotion efforts by people who are quite good (or at least relentless) at doing so.
People are going to discord for that because it is convenient as a platform for a variety of reasons, not because they want to hide it from the public imho.
Information organization is hard and lots of work. Not everyone (or every community) has the same need to organize information, and not everyone has the ability or inclination to do so.
Information getting lost on discord is a problem, but that just means the useful stuff needs to be archived elsewhere, and frequently there is another distribution network involved.