I think it happens because of reddit. A subreddit sucks compared to a forum, yet it's close enough that separate sites have an extremely hard time getting users due to friction. A discord is different enough that reddit isn't a substitute and most people already have accounts, while providing what reddit lacks the most - personal identity and human relationships.

I agree with all that, the issue for me is when I simply what to peruse info and am forced to join their community. An annoying case for me was a game dev that only posted parch notes in their discord as opposed to a dev blog or something.

I think for me a lot of the pains would be solved if discord allowed a guest mode to temporarily view a server without the (admittedly small) hassle of having to leave the server

You should see some of the highly automated, well-organized, well-run discords out there. It's pretty eye-opening and takes some getting used to, coming from a forum-centric world.

They’re different use cases. Maybe you replace your IRC or your slack with discord, but forums are a totally different thing.

Exactly, yet here we are.