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 is possible and commonplace in jobs where you physically work. Construction, any of the trades, factory work, food service... if you work 40 hours you have actually worked very close to 40 hours. There isn't any real opportunity to slack off or stand around in those jobs. Office jobs, yeah. Many of those are bullshit jobs.
From the outside, it seems like startups do this better than most.