Since no other comment explicitly spelled it out fully: Why don't people work nine hours in the USA?

The answer is very simple: Benefits, overtime, breaks, and other bits of labor law are written around an eight-hour day. This coerces employers and employees alike into preferring eight hours over seven or nine, simply as a matter of optimizing the business's need to pay out overtime with the employee's need to secure FTE benefits.

If there were such a thing as a 20hr work week in law, not defined as part-time with no benefits, but defined as part-time with partial or full benefits, then I imagine that everybody's tune would change quite quickly and we would end up multimodal, with one peak at four hrs/day and another at eight hrs/day.