According to the speaker, which one is NOT the real reason for American people’s unwillingness to take all their vacation?

At some point after the war Europeans decided to take more time off, while Americans opted (or were persuaded) to keep their shoulders to the wheel for longer. A survey by Project Time Off found that the main reason why Americans are reluctant to take all their vacation is fear of being replaceable. Other factors are heavy workloads and lack of cover from fellow employees. One useful feature of public holidays is that workers worry less about bunking off if everyone else does the same.
   Forgiving holiday time does not always please the boss. A study by the Harvard Business Review in 2016 found that those who took 11 or more days off a year were almost twice as likely to get a raise or a bonus as those who took ten days off or fewer (although the causation could be the other way around, star workers may feel they can afford to take a break).
   Nor do extra hours automatically lead to higher productivity. An analysis of figures from the OECD, a club of mostly rich countries, in 2013 found a negative correlation between GDP per hour and the number of hours worked across member countries. Again, the causation is unclear — workers in richer nations may feel they can take more time off. But there is plenty of other evidence. A study of munitions workers in the First World War found that their output per hour tended to decline once they spent over 50 hours a week toiling. The Institute for Employment Studies in Britain reviewed academic research on the subject and concluded that "long hours working (more than 48 hours a week) was associated with (but was not proved to cause) various negative effects, such as decreased productivity, poor performance, health problems and lower employee motivation. "
   The danger is that long hours simply lead to wasted effort. C. Northcote Parkinson, a management theorist, wrote that "work expands to fill the time available". Every editor knows that many journalists only deliver their articles when the deadline is imminent. Workers may be staring at their computers, but how many have disappeared down the rabbit hole of Twitter spats?
   The modern world is supposedly moving in a direction where routine tasks are automated, leaving the more creative processes to the humans. And humans are undoubtedly more creative when they are not feeling tired.
   So enjoy the break, American workers, and maybe take the next week off as well.