Well this is one of those answers, at least from my perspective, that is: "Well, it's always been that way..." or so it seems from the span of the last 5 years.
If I had to suggest a guess though, I would say that it's pretty consistent that crafting
supplies, ingredients and intermediate products are intentionally made to be non-stackable, as it's consistent through almost all areas of crafting (with only a few exceptions to this as a matter of compromise, such as when a spell component is also a CNR ingredient).
The reason for this, I would suspect, is to help throttle the rate of crafting by putting a practical limit upon how many CNR-related items one can carry at a given time. I don't know if that's the actual reason or not, but it seems to fit the pattern.
To put a more mechanical/technological spin on it, the "base items" that those things are based on are not stackable in the NWN engine, and at this point, refactoring them to something that is stackable would be a pretty hefty effort.
Anyway, possibly not the answers you wanted, but there it is.
