I don’t mind Discord for smaller groups of friends I know in real life, but larger groups end up with so many notifications that I end up ignoring the app. A forum would feel a lot nicer, like it’s not a chore to keep up with things.

Agreed. In general people seem to like centralised platforms. They don't want to sign up on another site for a specific purpose. They stick to what they know unless there's good reason to change (mostly peer/ad/social media pressure I feel like).

In a way Lemmy is similar in that it's a single platform to access all types of content. Given most people don't care about the technical "how", I can see why they like Discord and Reddit.