Ghasts naturally spawn in massive numbers in the new Soul Sand Valley biomes. They don't seem to make any appearances in any of the other nether biomes, not even the classic Nether Wastes. Gamerule doMobSpawning is true, and this is all on a singleplayer world. I've attached a few images showing about 6 ghasts at once, as well as a video where they moan like crazy.

NOTE: I have searched for this issue to see if anyone has already reported it, and saw nothing, at least not related to the Java Edition.