There are actually several reasons for this, based on performance issues. The principle reason is for the desire to have "level frame" when at a rated load. This keeps your load in the bed if the tailgate is open (which is common for 'builder loads' (ie lumber, shingles, long stuff, etc). Another reason is to have a soft initial ride rate for when you are empty. That includes partial fuel load. The springs are nonlinear in the rear: the more you load it, the spring RATE (lbs/in or N/mm) increases. Steel springs soften with use (yes they do) so the goal is to have a normal operating spring height when there is 20 - 40k miles on it.
Lastly, there is the SEMA effect (Specialty Equipment Manufacturers Association). They demand there be provisions and dimensions for aftermarket parts. This include oversized wheels and tires, camper boxes, light bars, snow plows, running boards, exhaust stacks, trailer hitches, etc. There are those owners who desire putting on the lift kits with Big Mudder Boggy Woggy Oscar Meijer tires with reverse wheels. They HAVE to fit into the wheelhouse without unfriendly tire contact by court order. A few years ago, I remember GM was sued by SEMA because it wanted to modify the 'standard' pickup box dimensions to accomodate larger loads and lumber sizes. Oh, NO claimed SEMA, that would put hundreds of cheezy camper box makers into bankrupcy because they would have to retool all their cardboard campers. GM lost the suit and now all the SEMA aftermarket crap has to be accomodated by the domestics. There were no Toyota or Nissan or Honda fullsized trucks at the time so they got off. BTW: were you all aware that Toyota approached GM to provide V8 motors for Tundra trucks? That's because they wanted to avoid having to pay for development and tooling for a truck motor. GM said NO and the now famous Toyota cast graphite iron V8 appeared. NASCAR fans ought to be able to tell you the advantage of a cast graphite motor over a conventional iron block. (No it's not pure graphite, its an expensive high carbon content iron, with huge cooling advantages when they drop the green flag). Not the greatest for longevity, though (just ask Kyle Busch, Denny Hamlin or Joey Logano).
Regardless of what you may think, the largest fraction of the GM pickup truck market is actually geared to the home construction segment of the economy. When pickups start selling again, that means housing starts are increasing. Don't ask how I know...
