40 is probably the most optimal way to extract the most value out of the most people. If only some worked less, there would probably be bigger HR overhead, like you say, plus those individuals would be less communicable. In reality, those companies who extract the most value from their workers are able to outcompete others, so the system stays. If company would pay less to workers (for less time used, half of "normal" employment), they would try to compensate with getting another job just to support themselves, because most workers are typically always on edge of supporting themselves (they move that edge whenever they get more money). If companies paid for less hours, their products would cost too much and couldn't compete with other companies.