Because this is what I call a “Caste Class” society, for the very reason that one’s caste status decides whether one can become an upper class/untouchable class or not. The right of the upper class/caste Hindus is to get unfree labour for their production and to keep their body and space away from ritual purity, while the untouchable class should provide only unfree labour and their only duty is to live for the Caste Hindu class and keep them away anything that would make them ritually impure.