Well if you start it and go on a quest does it really matter how long it takes? I mean you aren't spending that extra time gather resources, so that is where the time comes in. I think for game balance it's probably like this. They have to find some way to offset the gains or no one would craft it the other way.
This sets you up for a choice. Horde night starts in 3hrs. Do I craft it in a stack and take longer making me rush to make bullets, or do I sacrifice resources for time gained to make those bullets.
To put it another way. Imagine you are making a pepperoni pizza. You can be precise and place every piece on one at a time perfectly, or you save time and just throw a handful of pepperoni at the pizza. Sure half will land on the floor, but you got the job done faster lol
Faster in any manufacturing setting is always sloppier and more wasteful. Precision measuring takes more time than eyeballing a handful of material quantities. I would guess this was their line of thinking.