Your point is well taken that 8 feet is wider than a deck needs to be.
I belive that the dimesnion was specified simply because American building material come in 4' x 8' sheets and 4 feet seemed too narrow. It probably seemed like a waste of material (and effort with a hand saw) to cut the sheets smaller than 8', so that's the size.
Of course this makes the deck bulky and heavy, which is a larger burden as particiaption declines. Real aircraft carriers came in many sizes, including some really tiny ones. Personally, I would prefer a 5' or 6' wide deck with the specified 4" understructure, as opposed to a full size deck layed out on concrete.