In the Overworld preset in SuperFlat worlds pillagers don't spawn near outposts.
What I expected to happen was...:
I expected pillagers to spawn near the outpost, so I can get Bad Omen effect and start a raid in the village nearby, but the pillagers never spawned. If I go to an outpost in a default world they spawn normally.
What actually happened was...:
Pillagers never spawned in the place, even if doMobSpawns command was set to true, the difficulty was Hard. I have only tried this in 1.17.1 and don't know about other versions.
Steps to Reproduce:
1. Start a new SuperFlat world and select the Overworld preset in Customize.
2. Locate a pillager outpost.
3. Pillagers don't spawn. Never Ever.