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.
That doesn't sound right. Each Discord community is its own separate space -- you still need people to join your specific community regardless of whether it is hosted on Discord or something better.
> though I still miss phpBB.
It hasn't gone away -- the last release was on August 29th, so this is still very much a viable option.
So communities being separate "spaces" doesn't create any meaningful friction with regards to adoption.
Discord is NOT a benefit. Its not publicly searchable and the chat format is just not suitable to a knowledge base or support based format.
Forums are much better in that regard.