Humans were still not only the cheapest robots around, but also, for many tasks, the only robots that could do the job. They were self-reproducing robots too. They showed up and worked generation after generation; give them 3000 calories a day and a few amenities, a little time off, and a strong jolt of fear, and you could work them at almost anything. Give them some ameliorative drugs and you had a working class, reified and coglike.