Add 8 hours sleep plus 1 hour for dinner plus 1 hour getting up and ready in the morning, that leaves you with 4 hours for your life during the weekdays. Then you watch an hour of TV with your significant other, now you've got 3 hours left. It gets pretty hard to pursue a side project, art, or a hobby living like that. Can you cut an hour or two off sleep? Maybe, but I didn't factor in things like working out or calling your friends/family so it's probably a wash.
What's more is that the lifestyle I've described above doesn't leave people with much time to consume products/services. If people had shorter workdays they would actually have time to spend money, which you'd think capitalism would want most of all. Cutting down the workday would save people so many pointless meetings as well since managers might actually feel some sort of constraint on the time they have their employees in their offices.
I think most people in tech or high-skilled white collar jobs see how their fellow employees do various things to reduce the 40 hour workweek to something reasonable. People work from home one day a week often, or they work four days a week supposedly doing 10 hour days instead of 8. There's a lot of charade involved in the 40 hour week for many employees. However, many other employees aren't able to find these loopholes and it results in a real grind...especially when the job isn't just 40 hours a week and turns into 50 or 60, which quickly becomes unbearable.
We've really got to stop this madness. If you work at SpaceX or Blue Origin, then yeah, what you're doing is pretty important and you might actually save the human race, so you can feel good about those 40 hours. But if you're working at an ad agency making gum commercials, let's get real, it could and should be done in 4 hours a day. And besides, SpaceX and Blue Origin can double their staff if needed to offset the shorter hours. Since their employees would be better rested and happier they probably won't even need 2x their staff, maybe just 1.5x.