Too stupid to know better?
Frederick Taylor, father of 'scientific management', testifying before Congress a hundred years ago:
'I can say, without the slightest hesitation, that the science of handling pig-iron is so great that the man who is … physically able to handle pig-iron and is sufficiently phlegmatic and stupid to choose this for his occupation is rarely able to comprehend the science of handling pig-iron.'
If you treat your employees like mushrooms (keep them in the dark and regularly throw crap on them), it's entirely likely you will get precisely the work you deserve in return.