Because Discourse users aren’t active users in the meaning they would be actual actors. Categories and tags are. And yes, that is a huge issue, and no, AFAIK, this is not a bug, but a Feature request, because it would change how Discourse works with AP.
Pure guess, but nowadays system makes possible how Discourse/Discourse works and it has been more important than how Discourse is federating to Mastodon direction. Discourse doesn’t understand Friendica either, or Pixelfed (well that one would be difficult, because content is so different and Discourse can’t federate images).