US gear makes R&P sets here so its not like no one does. Also, the process of cutting the gears is done on automatic equipment so there really isn't much manual labor involved. One of the bigger companies that makes the equipment that cuts the gears from the castings/forgings sells machinery to them (US Gear) as well as a couple Korean companies I looked up. Since none of the standard gear sets and other parts from other D44 designs are compatible with the JK D44 housing, it could be a volume thing?
This is probably more information than anyone really gives a damn about but now that I started looking this shit up I might as well see it through...