I'd venture to think they did actual testing to determine where insulation should be applied. There's a science to it, but total blanketing of a panel may not be as effective as just isolated tiles. The outside noise you hear on the inside is from the panels resonating. Stick some tiles in some strategic areas and you break up the resonant frequency(ies) of the panel. Total blanketing of a panel would effectively just make a thicker panel that would just resonate at frequencies higher or lower than previously. You'd perceive a change, and it may be satisfactory results to you, but it may not be as efficient as it could be and you could be wasting a lot of insulation. Insulation adds weight and costs money. If GM were to just haphazardly slap a bunch of it in every Escalade, Denali, etc. ever produced, it'd add up to huge unnecessary manufacturing costs and extra weight. I believe the factory sound insulating can be improved as the engineers have to strike a balance between cost and effect. As an average Joe Car Owner and non-acoustical engineer with no measuring equipment, the most we can do is apply additional insulation with an educated mindset.
My idea for when I do mine is to blanket the more offending panels such as the doors, floors and ceiling with some thinner stuff to first lower the resonant frequency and increase the thermal insulation aspect. Then, I'll apply thicker/heavier tiles in the wider, lesser-supported areas to break up and dampen any remaining resonance.