You're looking at the automakers from the wrong perspective. You're thinking that'd they'd want to optimize their product to have the most efficiency/power. However, the automakers are primarily watching their bottom line and ease of construction. Dual pipes can give more power, but are lost on an engine struggling to breathe through a 2 barrel carb. They're also more expensive, and mean you need more hangers, gaskets, etc, which take more time, which means more money still. Now, if you go dual exhaust, 4 barrel and a new intake, you'll be more efficient and make more power. But you also spent more money! Now, maybe the factory "optimized" the whole package, given that a restrictive carb and intake won't need a dual exhaust. Or maybe they didn't even do that, just based it on how much it cost because they had set the price point before they built the car. Needless to say, its unlikely that the factory had the same goals in mind as you do.
