Why is this the case, out of curiosity?
It feels like it’d make more sense if every Discourse user was an ActivityPub actor, and forum categories were ActivityPub groups that reposted each user’s posts in that category. This is also how Lemmy works, I believe.
Right now if I follow a category on Mastodon there’s no indicator of who posted what, which is pretty undesirable for my community because anyone on the internet can post arbitrary content to an official looking ActivityPub account on our domain.