The nine to five, five day, working week is a modern construct. It was introduced by Henry Ford to his factory workforce in 1926, and become common practice in the US (and subsequently other parts of the world) after The Fair Labour Standards Act of 1938.
Ford favoured a shorter week (until then, six working days had been the norm) for purely commercial reasons. He was losing good employees, and found that a 40 hour week, worked in shifts, was the best way to optimise workers (who needed to be kept alert and motivated) with expensive machinery and power plants, which shouldn’t be left idle for too long.
Most of us in the developed world now live in a post-industrial age where knowledge-based products and services are replacing manufacturing. If your work can be completed over an internet or telephone connection, it’s less likely that you physically be in the office, or work specific hours. Yet we still rely on a way of working that was designed for the factory production line.