The “commonish” reason I see for this is that if you enable embedding at the moment on a giant site we create a mountain of topics with zero replies, cause topics are created when you visit the “embedding” page.
This would help this particular case, even if we optimise our flow.
When you click “add comment” on the blog/wiki etc. we have no choice but to create a topic on the Discourse site, regardless if you go ahead and actually add a comment.