Lush caves and dripstone caves will not be generated in the Single Biome world

Lush caves and dripstone caves will not be generated in the Single Biome world.
Lush cave and dripstone cave biomes can be randomly generated underground in all biomes in the Default world, but do not do so in the Single Biome world.

Probably because the Single Biome world still uses a 2D biome world generation instead of a 3D biome world generation.