It's all in the odds, the odds are that you will get a weapon or an item from X chest 100% of the time. The odds of a specific item never change. In terms of primals the odds are 1/9, of any specific weapon dropping. And the odds never change, no matter win 9, 90, 900. So it is very possible with the odds never changing to in fact never see an item drop ever. It's like a container filled with colored tags, you draw a tag, and the odd is 1/9 if it being the color you want. Then you put the tag back in, shake, and draw again. The odds are again 1/9. I have done garuda and ifrit over 100 times each and i have not see all the weapons drop yet. And i may never see them, the odd of 1/9, is not the same a 1/9 percentile.

This is one of the game models i hate, because you are not guaranteed your item after the 10-100 or 1000th run. I have seen people get item X 1/1 kills as hey hit gold on the odds, i have seen people 0/100 with the same identical odds.