@honeyed_coffee For the reasons the OP mentioned. Familiar faces, being recognized in a community instead of being just today's main character.
In a single large forum most participants are silent, as they must be or it'd be a cacophony. Many are silent out of worry that they need to say something good enough to impress a hundred thousand people, not just something interesting to their local 100 friends.
On Fediverse things escape their local instances and their local forum-groups by boosts mostly.