Every atma is different so to make them stackable you wouldn't know which atma you have and which one you still need. Also if it's a key items you could only hold one of each at a time and not get multiples until you finish the weapon. Im halfay through my atma I'm getting for ninja. It would suck that if I wanted to do another atma weapon b4 ninja is released (which I did plan on) that I would have to turn in all 12, forcing me to farm another 12, which I probably won't have time to finish b4 ninja comes out since my overall atma average is one every 72 fates.