Dungeons have worst loot table in game. There are 13 potential weapons that can drop from 3 different sets, so 39 potential weapons. Plus, only a 50% chance of even getting a weapon because it is 50/50 weapon or jewelry. And it only drops from final boss. So 1 / 39 is about a 2.5% chance, but cut in half because of chance to get jewelry, so about 1.25% chance of an Undaunted Infiltrator Lightning Staff. That is an ugly 98.75% chance of not getting an Undaunted Infiltrator Lightning Staff. And you only have 3 other people to share with, not 11 other people like in Trials.
Is it RNG? Yep. But it is also a rough loot table. You are basically rolling a 78-sided die and only 1 number will get you what you want.