Most of these communuties using Discord are better served by something that isn't a chatroom. So, so, so confusingly many of them use them as a store of permanent information. Like a website+forum.
Many times the benefit of Discord is the ability to paywall parts of it with Patreon integration. We need more foss and federated options that do this.
It's harder to see on a large Lemmy instance like LW, but most of the fediverse is very patchwork. The network of Lemmy sites is itself very patchwork, with the MLs, Hexbear, Beehaw, NSFW, etc. all having different defederation profiles, but the whole space is an incomplete mesh. Mastodon has more themed instances than Lemmy, more very small instances than Lemmy, and a much bigger anti-capital, anti-commerce bent than Lemmy, with many more people complaining on main about other instances rules and federation policies, so if you look, you can really see it.
But the whole fedi project is patchwork by nature.