The government being involved is the most likely answer as to why a 40 hour work week is standard and why there are so many "minimum wage" jobs. A funny thing happens when you set floors or ceilings in markets, you hit them. There should be no minimum wage. People excepting jobs will set the minimum wage because people act in their own self interest. If the pay is too low, no one will take the job. This is how labor markets are set everywhere else in the economy, I don't understand how the bottom is different.
The government shouldn't have anything to do with the work week, the market should decide because what works in one sector in one region may not work in another. The government isn't as smart as the entire labor market making decisions for themselves.