Due to changes in the spawn rules around Nitwits they became uncommon/rare in general, this must've happened during the 1.14 update, that changed the whole trading, look and interaction of our big nosed friends. So not being able to get these 2 types of nitwits has been an issue since april 2019.

Generated villages will spawn Nitwits but since Jungle and Swamp villages don't exist by default both type of Nitwits are simply said extinct in Minecraft!

I guess every new mob costs us an old one?

How to generate this bug or lack of nitwits; Open a survival world and try to find a jungle or swamp village, spoiler, they don't exist.
Breeding doesn't spawn nitwits anymore, so building a breeder in a Swamp or Jungle biome won't give you said nitwits nor any other.

This lack of nitwits has been the case since 1.14, eventhough Nitwits may be useless to some, especially for players who don't know the game from earlier days, that doesn't mean an existing mob should just be deleted or suddenly become rare. 

Suggested solution: Simply add nitwits back to the breeding system.