The question of why we still work 40 hour weeks under later capitalism is kind of like a milder question of why entry level jobs in consulting and finance have such long work hours, why not hire 2 people for cheaper to each work 40 hour weeks?
Context is that which is scarce explains why some fields like consulting and finance have fewer people working longer hours vs. more people working normal hours. This seems kinda obvious in retrospect but it says something interesting that it wasn’t obvious while I was in uni.