Triple Triad NPC Reward % drop rate?!!

[Aeyis]

That's the nature of RNG, which is why a VERY large pool of players

Square's SD on their RNG is absurdly large. It takes a very large pool of players indeed.

The drop rates seemed pretty generous imo

Better then trial or NPC rates for sure. However that is likely due to them being 1* cards. That said, they are not as generous as you believe.
It still took me 6-7 dungeons before I even saw one drop. Had to do some of the other dungeons 4-5 times to get their cards.