I also hate this - I think it is mostly caused by people wanting a "pretty" web site that will look the same for everyone. (mostly) graphics people design these kinda things to fit the lowest resolution (currently 800x600), and probably use a program like dreamweaver to create the HTML - any page could be designed without complicated coding to fill the whole screen - generally forum software like this is coded by a nerdy geeky guy who is running 1600x1200
and wants to use all that space.
I don't think you can do anything to fix it.