This occurred to me as well. A positive is that many of the main site’s links are already posted in the forum and the main site’s link is in the pinned welcome statement, all of which are searchable.
What confused me until today is that when you scroll down a thread on the desktop version of the forum, the header turns into the title of the thread. This behavior doesn’t happen on mobile. The confusing part is that at first it seems like link back to the index is gone. But it is just truncated to where the px logo is.