Compared with Europeans, Americans are more likely to be employed and more likely to work longer hours--employed Americans put in about three hours more per week than employed Frenchmen. Most important, Americans take fewer (and shorter) vacations. The average American takes off less than six weeks a year; the average Frenchman almost 12. The world champion vacationers are the Swedes, at 16 and a half weeks per year.
I’m not sure if the entire Forbes article answers the question of why Americans work harder than Europeans. I believe the cultural aspect may be more important than tax policies.