Because of the competitive nature of business (and humans).
If you can build a factory that can build products more efficiently, it means you can sell them cheaper then your competitors, meaning you can move more volume and thus make more profit. So you are definitely not going to run that new shiny factory for only 3 days a week.
Eventually, competitors will notice your improved output and react with their own improved factory. If you sit still in your shiny new factory, you will eventually be outpaced by competitors as they catch up.
In the end though, the throughput per human is greatly increased as computers and machines do (most of) the work. This has increased prosperity greatly.
Unless you can accept living with less then the competition, you are not going to work less then your competitors.
Nobody is forcing you to work a 40h week, but because 40 hours is about the average of how many hours people work, you'll need to match that to stay 'competitive'. It's a very primitive instinct in natural selection.
For people earning average wage, they often cannot afford to work less hours, as prices of commodities (housing, food, energy) are naturally regulated by what people can afford. Because most of their peers work 40 hours, they need to do the same in order to be able to pay their bills.
Those that make more than average wage could afford to work less, yet most people chose not to do that. Because now that you make a lot of money, you could still work 40 hours and get a bigger house then your neighbor! And a faster car! You could impress a potential partner with that, which increases your chance of reproduction and creating (w/h)ealthy offspring.
No amount of automation, computers, machines or whatever technology will prevent people from staying competitive.
edit: replaced 'minimum wage' with 'average wage', because cost of living is highly location dependent.
People making minimum wage typically have to work well above 40 hours, and even then often find their earnings difficult to live off of. See also: all the calls for paying low-level employees a "living wage".
