I doubt it's guaranteed. You're just getting unlucky (also don't forget the drop rate percentage increase). Raids are not meant to be easy, you're not meant to be getting items every 10-30 chests. It's an incredibly rare chance that you're going to be getting a drop so why that you're 70 chests dry are you implying it's an issue?
You're playing on the Normal mode which reduces the chance of you getting a drop. Raids are not a one-sided benefit to any mode but just remember what you're playing on and the difference in drop rate percentages. The economy is harder for a reason.