People use Discord because it's useful and they like the experience. Discourse is nice for purely written communication but if you want to setup a call, direct message and, more importantly, write bots (presentation, topic organization, news, modding, now even AI Q&A); people will fall back to Discord.
Pointing at Discourse when it doesn't have the same value proposition and blaming users is just not useful at all.
Of course, this isn’t always the case, your messages can still go unanswered for days. But in my experience, when I see that a project has a Discord, I now think "Great I might get an answer". And this doesn't dismiss all the cons that others have already mention
Look at the VIM help as an example of great manual. Or the GNU software documentation. Now, you see “Join our Discord” instead.
The UX on Discord is excellent. That's why so many people, including nerds that used to happily use IRC and Mumble, use it. It's easy to create your own server, administer it with granular permissions, and to invite your friends to it. You can instantly start voice and video calls with screen sharing, for free. It's also a live chat with excellent formatting capabilities and has multiline support, unlike IRC (unless this has changed recently?), which makes it much better suited for technical questions involving code. Oh, it also has actual code formatting built in so it's actually legible (though this remains a feature underutilized).
The problem with discord is it's just not visible outside of discord, it can't be indexed, archived and searched through other means, and is vulnerable to the whims of the server owner. It's also not very good at being a forum replacement because its threading capabilities and UI wasn't designed for that purpose, so it feels bolted on (because it is).
[0] hacks aside
People on this site really confuse this issue: your dislike of Discord is an ethical one, because it goes against what you believe a chat should be (free/open/whatever). Most people unfortunately just don’t give a shit.
As far as I know there is no other voice communication program that handles voice channels TeamSpeak style with a chat box next to it. If there is one I'd jump ship.
That said, most people use Discord because other people use it and there is much less friction to add a new "server" (i hate how they use that term instead of something like "community" or whatever) alongside those you are already part of than making a new account to yet another site.
I know i'm using Discord mainly because the people actually go there. I used IRC since the 90s but for more than a decade it has been channels with 15-20 people all of whom are AFK and idling via some proxy bot or whatever with the occasional chat now and then by a couple of people (Discord also has a lot of idlers but it has 10x more people too so that couple of people becomes 20 people). IRC not having server-side storage of discussions does not help because i can't login to a channel, ask/answer a question and leave later. I know some people like that lack of permanence but personally i find Discord much easier on actually making or even starting discussions (e.g. i might respond to a message someone left 8 hours ago which is seen by someone else right after i post it and starts a discussion - this only happens because i was able to see that 8h old message). I know IRC supposedly added optional support for this recently but i don't know anything that uses it (and even then it wont solve the "people go to Discord" problem since other services like Matrix already supported that for years and people do not go to Matrix).
Let's fix that.
Discord-using people use Discord because it's useful to them and they like the experience.
Having said which, finding out is obstructed by the fact that very few organisers offer users both.