That certain Demiatma only drop from certain FATEs / CEs adds another layer of RNG on top of the odiously low drop rate; you have to luck out twice to get one. Overworld FATEs let you target colors, but the drop rates there are no better than in South Horn (maybe worse) so either way people aren't getting the same return on time investment through no fault of their own, or even other players. It's just luck.
I noped out of South Horn after hitting Knowledge 20 and none of the FATEs / CEs I needed were spawning. Still needed 2 green, 1 light blue, and 1 yellow. Grinded in Yak Tel for 2 hours, got 1. Going at that rate it's going to take me as much time to get the rest of the ones I need as it took my brother to get twice as many. Feels bad.
And the fact you can't buy Arcanite Ore until you finish the Demiatma grind disincentivizes playing any other content since you'll be wasting tomestones you could use to buy more otherwise.
I want to go exploring in South Horn like you're supposed to and just do the daily dungeon grind, but RNGesus isn't favoring me.