Earlier quoted context omitted.
I am not going to deny anything you said, its almost 100% true. But I believe your missing an important "fact"(maybe). That is its not really physically/physiologically/mentally possible to put in 40 hours of true "work". Not consistently anyways. So as much as it is we not truly putting in 40 hours, its the entire effort to be available for 40 hours IMO that you are getting paid for.
It's most definitely possible to put in 40 hours of work. This mainly happens in more labor-intensive jobs. Note that it doesn't have to be construction work. Even your primary care physician's job might be described as "labor-intensive," just because they have so many patients to meet with and so much administrative work to do for each one. I'm sure many (most?) doctors do put in a legitimate 40 hours of work a week…
Beste would be I work at home so during the day time I can take a nap. The kid wakes up early. Bring him to school. I take a nap. Pick him up again take some time out with him and when he sleeps I work. But because I have to sit at work I have to work during my least productive time. Because I don't have the energy to do 24/7.
So before marriage and kids I had 2 responsibilities. Going to work and actually working. But after marriage I had more and I couldn't cater it.