1) I personally need a certain amount of focus to be really productive. Pure focus on one thing with enough "cadence" brings me in the zone. 2) Employers often have the idea as well. Although part-time jobs grow year-over-year, the ones getting promotions are still usually the employees dedicated 40 fulltime. I don't know if this is just a matter of perspective or based on tangible results (e.g. proven better productivity). 3) The "10 000 hour rule"

I should emphasize that the 40 hours should be spend well and the employee should use enough time for relaxation, rest and leasure. 40 hours for e.g. a head of family in busy waters may quickly be detrimental.