Why would bike-packing gear help you climb faster? I suppose racks and heavier panniers would make a small difference, but aerodynamics would be insignificant.
simple, less volume = less weight. I had more space and being in Italy I couldn't help but carry a new bottle of wine to try in each region. You can pour it into a platypus pouch and leave the glass but it's still an extra 750mL of liquid separate from my main water supply. Same with every new local cookie and salami etc. Cargo always expands to fill available space