I highly think this is due to the culture around work and the value system found in the workplace. Often times, the worth of an employee is measured by how much they can output/get done. So, in a competitive environment where employees are trying to stand out and be seen as the cream of the crop, it is natural for people to start working much more than the standard 40 hour work week. Being seen as busy and producing much for the company makes other employees want to do the same and out do one another. This kind of environment can vary from company to company, even within different departments within a company. It very much depends on how the leadership sets the value system and how they lead by example. I have worked in Supply Chain in my company where you are expected to do more than 40 hours and to be available/responsible for work even after hours. Managers, directors, and more of the leadership were constantly up late at night or working through the weekends and expected their reports to do the same. Within the same company itself, when I transitioned to R&D, the culture was almost flipped; while there was still the expectation to finish work, it was not to be at the expense of your life outside of work. 40 hours was what you should do, and doing more would be at more exceptional cases, and taking breaks/vacations were more encouraged. This is exactly what the leadership there did, and they held their reports to that standard as well: having a more robust work-life balance.