I don't think people who choose Discord necessarily care about that. Discord is where the people are, so that's where they go. It also costs close to nothing to setup a server and since it has a lower barrier of entry than hosting your own forum, it's deemed good enough.
That said, modern forum software like Discourse https://www.discourse.org/ or Flarum https://flarum.org/ can be pretty good, though I still miss phpBB.