Why? (I have a guess)
While it might appear that it's deliberate on the part of H-D to create sales for Screamin' Eagle parts, I think the real reason is that corking up the bike is the only way they can meet the DOT/EPA sound regulations. Air cooled engines naturally tend to produce more mechanical noise, which adds to any intake or exhaust noise. And styling requirments dictate air cleaners and mufflers of minimal volume. It is possible to flow a lot of air without making a lot of noise, but it requires a large volume airbox and large filter element, and large volume mufflers.