Not only the extra parts cost, there is the extra labor to press in the bronze bushings.
When you are talking a couple bucks on 1,000's of units it makes a big change on the bottom line.
I did the upgrade myself. Over two days it took 11 hours including jacking the trailer, getting tools out and putting them away, and removing the OEM parts and putting in the Dexter wet bolts, bushings, and the EZ flex equalizer. I am sure at the factory it would add less than fifteen minutes to do the entire thing, when the frame/springs/axles are easily accessible, nothing to remove, and it is on an assembly line. I can't defend the manufacturers like you are. Putting something together that has a life of a few thousand miles is simply wrong.