Originally posted by Chicken
If they all look the same, then it is probably because no one has done much to it, or not enough. I don't use phpbb, but I'm guessing that it could be customized and made to look completely different by a competent designer. I've seen vbulletin forums that I couldn't tell were vbull, and other forums that were done well.
I think this is more of a case that people tend to do small mods to existing themes and styles you've seen before (the stock ones, or more popular themes, etc.).