The 40 hour week is also enshrined in law because our labor laws are codified around the ideal of factory jobs. If you want a job that has benefits then that's

The 40 hour week is also enshrined in law because our labor laws are codified around the ideal of factory jobs. If you want a job that has benefits then that's a "full time" job at ~40 hours a week, a 15 hour/week job is considered to be a completely different sort of beast by the government (something more akin to a temp summer job).