Pillager Outposts doesn't spawn on superflat worlds

When I create a new superflat world using default preset, pillager outposts doesn't generate naturally. The command "/locate structure minecraft:pillager_outpost" doesn't work either. In a default world in plains biome villages, pillager outposts and ruined portals spawn by default.

 

Steps to reproduce the problem:

1. Create a new superflat world using default preset.

2. Try using /locate structure minecraft:pillager_outpost

3. An error will appear saying Could not find a structure of type "minecraft:pillager_outpost" nearby