In Ram and then Ford's stiuation, the increased tow ratings had more to do with improved frames than anything else. Ford could not match Ram's 30K figure until the new model and its new frame were introduced.
I suspect the GM cousins are in the same boat and maybe as another poster wrote, they want to be in a certain niche and dont care about that tiny percentage of people who buy 30K# tow rigs.
That probably makes sense because Ram and Ford sell very few of those high rated combinations compared to their total truck sales.
