I agree with srg36 — I believe that its primarily due to peer pressure. Americans are vastly overworked. According for the Center for Progress, Americans are overworked in several ways:
1. We have no maximum work hour limit. Although this might be beneficial to a point, it also does not punish companies that overwork their employees.
2. In the US, more than 85% of men and 66% of women work more than 40 hours a week.

In addition to that, not all companies grant sick days or personal days and even vacation days are often docked.

All of this, in combination with our sorely lacking maternity and paternity leaves, simply means that our workforce is overworked and under motivated to get the job done.

I believe that less work hours would honestly help us and probably make us more productive as a nation overall.