I think it's due to how Discord evolved as a platform

Discord start as "your private place for your friends to talk" during a time where there were a lot of privacy issues with other communication methods.

Then as it grew beyond this scope of being a private place for friends, it would have been good for indexing to be added but indexing a normal text channel is really hard since you don't know where the conversation starts / stops to submit to a sitemap.

Now we've got large public communities and forum channels so it's possible they roll out their own version soon, but it does still slightly go against how their product was originally created so there may be some hesitation with adding it due to not knowing what the community reaction will be like.