In the real factory is an all robotic workforce. You never see trucks deliver any supplies to the factory. Thus clearly all the factory's output is made from Oompa loompa clones. Brutally fed into the machines live when they come of age to extract the ingredients from their living flesh. All the candy is made from oompa loompa sugar which is highly concentrated in the oompa loompa body, and the coloration is from their hair, blood and bones.
The ones we see in the movies are simply the insane psychopathic ones that have long since snapped, surrounded by their fellows in candy and chocolate forms, knowing full well the horrible process that takes place behind locked doors. Why do they show no emotion, even when singing, even when horrible things are actively happening to the humans around them. They have personally seen loved ones fed into the sugar extractor, stared into their eyes as the machine ripped still living flesh from still living bones with blades dulled by time and felt... nothing....
* Greiger needs Dr Pepper to get through the working day.
EDIT: My sister told me something similar to that story the first time I saw the original C&tCF. It has become head canon all my life.