As an observation, I think a great deal of this is because of the "depth of features" of things a really good service (like Discord) gets insanely right. There's just this mountain of really important quality-of-life things, or needed features available out of the box.
For example, I was scared when I started using discord, because it's entirely reasonable to think if I'm logged in in two places and start a voice chat, surely it'll start playing that audio anywhere I'm logged in, right? Everything else follows a "you do it on one machine you're logged in on, and you do it on all the others as well (such as writing a partial message, and not posting it - the same partial message is right there waiting if you grab a different device).
Well, not only do they correctly handle that one use case (audio chats) with a totally different 'you're doing it only in one place' model, they even track which machine you're currently live on. If you're actively using your home machine, they don't ring your phone - your phone which you might have left home. Or the session you accidentally left running on your office machine. They know you're actually live on a particular machine, and don't annoy or embarrass people by making the others ring.
There are just a lot of really, really important things like this they get right.
Forums did have a little bit of shared standardization, since a lot of people tend to use only a handful of highly popular forum apps, but kinda like the old 90s-style thing where there was always a roster of "must have" OS tweaks and apps you'd install on a new machine (and get to thinking 'geez, why don't they just make this part of the OS) - there's a similar thing with forums where there are a lot of sidecar things that ought to part of every forum, but simply aren't standardized. (Some of these are critical to doing certain jobs; some of them are, like accessibility features, critical to certain users, and difficult to make forum admins care about if they don't personally have the problem).
That, and what you're saying about "having it all in one place". It has a lot of benefits, like being able to cross-communicate and deeplink between communities.