I'm not sure if it's impossible to reach the forum from the main website but, after a bit of browsing, I cannot find a link. The forum is mentioned on the "Contribute" page but there's no link to it anywhere that I can find.
The main website has been a contentious issue for a while. See this post:
http://forums.scribus.net/index.php/topic,2282.0.html to follow the most recent conversation about it. Or, you can have a look here:
http://scribus.martinreininger.de/index.html to see the - totally unofficial - results of that conversation (still a work in progress, but no progress has been made for a while).
If I was to sum-up the conversation, the gist of it would be: The current site is doing a poor job of promoting Scribus but nothing can change unless whoever administers the site decides to do something about it.
As for a link from the forum to the main site, I think you are correct in saying that there should be one. You should be able to easily navigate from the forum to the wiki and to the main site, back and forth in whatever order people find the sites.
There has also been discussion about the forum recently and you can follow that here:
http://forums.scribus.net/index.php/topic,2328.0.htmlI have no idea whether anything will change in the near - or far - future. The person/people who administer these sites have not been part of these conversations and I have no idea if they even know that the conversations are happening. Also, if the admins don't want to do anything to improve the situation then nothing will get done.