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.
As another example, when I was a college student there were definitely a lot of weeks when I put in a legitimate 40 hours of work. But it was distributed over many activities, from actually sitting in lecture, to attending office hours, taking tests, studying/reading, working on homework, and participating in extracurriculars. There was a ton of downtime mixed in, and there certainly were virtually no solid eight hour blocks of nothing but work.
I honestly think a major impediment to office workers like programmers getting in a full 40 hours of productive work is just the environment itself. Just remaining stationary in an office for 8-9 hours a day is intrinsically exhausting, not physically but mentally. The worst part is when you have downtime, but still must remain stationary at your desk in the office. It now feels like you're working, but you're not. Your energy is draining, but you're not actually doing anything productive with that spent energy.
(To be fair, I also frequently would do homework up to or beyond midnight, or even on the weekends, whereas today I very rarely do work for my employer after I go home, unless an emergency comes up or I just honestly am so interested in a project that I want to. The upside to the 9-5 workday is that it has a well-defined beginning and end. In college, there was never a feeling of being done, except maybe after your final final for that semester.)