What I mean is, Reddit to me seems a step closer to something like Discord. And yet Discord is apparently still necessary; so why Reddit, and not traditional forum + Discord?

I added 'single login' as an after thought, because it didn't occur to me at first, but I think that must be a big reason. But even then there's Discourse, with not single but much easier and OAuth (signup/)login.

The default state of a Discord is hidden. You need an invite link to join a "server." It is set up in a way that your meeting point is outside of Discord, usually real life, at least per their marketing material (irl groups are probably the most value in terms of ad revenue).

So, yeah, reddit is the searchability.

But again, it has nothing to add there over a classic forum.

Reddit has a standardized layout, you don't have to learn a new interface. It has single sign on, so you don't have to create a new account to participate in a community (low friction) and it has the people. Also, it has the voting concept which helps filter content so you aren't having to do that yourself by scanning topics.

A "classic forum" trying to get off the ground would be competing with all of that. You could also argue, "why Discord? Why not IRC?" for much the same reasons.

Yes, that is exactly what I suggested in comment previous to (up-thread of) the one you replied to.

As I said there, there is also Discourse, with only slightly more multi-community friction really, making it far easier and more standardised to do initial sign-up.