I think you're missing the point of a forum like discourse, which is most definitely not a classic forum.

In no way is it similar to lemmy or Reddit when it comes to functionality or the problem space that it solves for. It's essentially stackoverflow, for niche communities, do you really think you could replace the utility of stackoverflow with.... Reddit?

It's specifically tailored to q&A, knowledge sharing/archiving, and as a living knowledge base. It does an excellent job of that, with copious features built specifically to enable and support that purpose. Which both lemmy and Reddit lack, they aren't even in the same problem space.

The platforms that actually utilize discourse effectively are some of the best to work on. Similar communities on social media platforms don't have anywhere near the level of quality, engagement, knowledge, or problem solving. Even simple features that encourage engagement in months or years old threads are massive boon.

It really is not much of a step to take your logic and replace lemmy or Reddit with Discord as a Q&A support and knowledge base platform. Which we all know is a terrible idea, and largely leads to a loss of knowledge and destroys discoverability.

Overall I get the feeling that you may not have experienced what makes that platform powerful? Which understandably can lead to thoughts that it's "just another forum", and that it is supplementable with a social media platform (which in the reality of it is laughably bad).

Check out https://forum.babylonjs.com/ for example. The framework is an absolute pain in the ass to onboard too but because their forum is just so damn good, it was a breeze compared to others.