Why? I would rather have a single board solution myself. Keep tacking on other components and it can get a bit kludgey - in my opinion. Electrically I'm sure there would be more worries about noise and maintaining good contacts. A re-do of the layout could happen any time - I'm not demanding anything - just a suggestion. I could add about 15 different little boards to my A1000 too, and instead the GBA1000 is an awesome layout. And as Darrin just mentioned, some of these would find homes in older existing Amiga cases - or if in ATX layout in any standard PC case.
Again, I'm happy with the way things currently are - just bringing up some ideas.