Dungeons are not created upon world generation

Summary: Currently, it is not possible/extremely rare to find dungeons that contain spawners not located in the Nether; i.e. zombie, skeleton, and spider spawners.

Steps To Reproduce:

1. Ensure you have the "Caves and Cliffs" switch enabled.
2. Choose a difficulty such that monsters spawn.
3. Explore your world for a sufficient amount of time in y level less than 64 to look for dungeons.
4. Notice how after a sufficient time of mining and caving, you will not find any dungeons.

Expected Result: Dungeons are created upon world generation.

Actual Result: Dungeons are not created upon world generation.