Originally Posted by Honesty

The stock intake is not optimized for flow and only flow. It is designed to quiet the sound of intake air and to prevent the vehicle from sucking in water into the combustion chamber.

The reason why aftermarket cold air intakes increase horsepower (which is just flow) is because they have fewer restrictions and allow for a higher amount of cfms to come through the pipe than the factory snorkel/filter. One has a more focused purpose, which it accomplishes.

The length of the tube is overall the same or only slightly longer than the factory intake, but it is straighter, wider, and simpler. As a result, air flows through it more easily. Therefore the vacuum in front of the intake valves draws in more air.

Length isn't the ONLY factor here. The overall design of the factory intake is not optimized for flow. That's the problem.