Random Monsters Map Patcher – diverse neutral armies.

What makes them so different that regular creatures can be deliberately generated, but they cannot? Or is it because they are neutral?

Without placing random creatures, we won't get NCF creatures:
How can you get a Pirate by placing an Archer?
Mapmakers can initially choose not to place any "random creatures" on the map. Such maps are doomed to have no NCF creatures, or only a very small number of them.