Andy has said eldritch items only come from cultist drops. That's why it doesn't drop from toilet paper.
As far as never dropping, I'm not sure about the chance of getting the eldritch uniques. If there aren't enough cultist waves, the eldritch item chance could be low.
Ah yes, eldritch uniques drop just from cultist waves makes sense to me, thank you. Too bad that toilet paper won't provide those eldritch Items then. Another lockdown Item (possibly noodles from the abyss) which is working complementary for eldritch items besides the Toilet paper for Standard Items would be great then ^^
As I am still farming Azatoth cultists I try to have my cultist spawn Rate as high as possible so the spawn themselves is not the problem... unless the cultist spawn Rate is not increased when feeding eldritch items to my hungry mr. Iron. Still, i try to have 20-30% on field via the rare eldritches. Therefore I should habe the spawn Rate at min. 40%
30% bonus would give you 21% spawn rate. Not 40%. 0.16*1.3=0,208.
And yes getting unique eldritch items is quite hard. Necronomicon never dropped for me in any game when not using bookworm.( which were not that many though)
Update: After a couple of games Necronomicon dropped At Wave 350+ during use of Yig. Therefore this bug is no longer actual... wrong Alert /shrug/
Update regarding spawn rate: yeah I thought that the mathematics behind are multiplicative with the Basic Chance... i consumed all white / blue eldritch items with mr. Iron and Got 20+ rare items on the field when reaching lv 400... with this calculation (average) (20×2% blue + 40×1% white) via consumed + 20×3% +8% via purples on field I got a Rate of roundabout 39% if mr iron feeds are counted and 26% if iron feeds vanish...
But it certainly feels odd. I just finished another 500 wave game and got no necronomicon, but I always get the chainsaw. So I see where this is coming from. Maybe legendary drop rate is just worse than unique and combined with the restriction to only cultists waves, the odds are just not good for a drop.