There are fixed costs of working at a job. Commuting is one important example. Often so are certain work benefits like health insurance. But there are other challenges to adding an additional employee but not additional hours like managerial attention, scheduling difficulties and deadlines when people work less per week and where to put extra workers that are less tangible. Fixed costs create an incentive to do your working in a compressed part of the life cycle. In the presence of fixed costs, as people get richer, they wouldn't cut back on hours per day or hours per year even, rather they'd cut back on the years (starting work older and finishing work earlier). And that's exactly what happened in Japan:
Womenomics for Japan: is the Abe policy for gendered employment viable in an era of precarity?