Weight ratings on light duty trucks

I think the answer is also in part because the manufacturers are sourcing axles from suppliers and that means taking what is on the shelf and adapting it the application. To maximize production volumes they design axles to cover a range of vehicles. The vehicle manufacturer on the other hand may be specifically targeting a particular class of truck "light duty", "medium duty", "heavy duty", etc... They go to the parts bin and grab the axle that best accommodates their design criteria. Just because the axles they grabbed might take more weight doesn't mean they will. The vehicle manufacturer may choose not to take advantage of it because it might require larger brakes, beefier frames, and larger engine to pull all that extra weight around to fully utilize the ratings on the axles.

I should preface the above with I know nothing about the industry and for all I know, the real answer could be something to do with fairies riding unicorns...