Normally in a default world, pillager spawn as they should at the structure, Pillager Outpost.

When you add the structure into a customizable superflat world, however:
Example of a preset: (minecraft:plains;village,pillager_outpost)

Once you find the structure in a customized superflat world, pillager will not spawn at all!

Screenshots below are shown with proof that I have not been in peaceful mode and in what preset they won't spawn at all.

Presets I used:

Classic Flat
Redstone Ready
Snowy Kingdom
Desert

And all of the other presets!