Of course, there is a search function on the Discord, but discussions scroll so fast and jump from topic to topic that they can never develop the same level of coherence like on a forum. There can not be an equivalent of "wall of text post" on discord, you can not give someone an answer by quoting him sentence by sentence and then answering each, and all that in a 500+ word reply. I mean you can technically, if you try really hard and don't let yourself be distracted, but it's much harder than on a forum.
But since this is specifically about games, let me give an example to something relevant with the theme of this site.
I had a question about a mod for Warhammer TW3 that I downloaded from steam and I wanted to ask the modders. Steam page linked to a discord server, a huge server in which I barely found the channel for that mod only through using the search function. There I asked a question and for the first two days I got one random "no idea" answer from random person, but there were 50 other posts... comments... messages, however you call them on discord, in those 3 days and since my question didn't tag anyone, it was left unseen by modders (if any were online in those few days) and I had no idea who were the modders to tag them and there was no @modteam tag or somethg. In any case, unless I would post my question few times a day, no one would ever answer and no one who knew the answer would see the question.
Compare this with opening a thread in a subforum for a mod. Hell, even a mod megathread on a forum is more readable than any channel on discord.
Of course, another problem is generational. I had these discussions with 20 years old kids who think forums are slow and archaic and need things to move and something to happen all the time, they need animated things on their screen and sometimes are even too lazy to write a message but simply record it and send it as audio because writing is so old timey.
I'm too old for that
.