Well, I do see your point, though there are several other issues that has to be considered. For starters:
* If many boards are joined into one, not only does the boards join but entire communities are joined into one, and there will probably be many voices raised against it (aswell as raised for).
* The bigger the board the more to maintain.
* If one big board goes down, so much is lost, if one of many goes down, there are still several others left.
* All the boards have different ways of dealing with things, they also have different approaches, discussionwise, rulewise, membershipwise, manyotherthingswise... the English Amiga Boards, for example, are nothing like these boards, and I do not think the members of that boards would be willing to have an Amiga.org like major board, just as little as the members of Amiga.org would like to see these boards transform into something like the English Amiga Boards. Finding a solution to that would be hard and I think that having a variety of boards to chose from is all but bad.
It would be a nice thing, I do admit that, but I think that it in the end would be a very troublesome affair. Probably more troublesome than pleasant.