'why does everyone use Discords instead of forums these days?'
idk, because running a forum largely costs money, and basically all the big forum software options have had various horrifying security issues over the years? (jk nobody pays attention to security issues)
It's not that Discord is good for this, it's that Discord is free and easy.
It's the same reason why personal fansites have been replaced by ad-infested fandom.com wikis.