Graeber asserts it's specifically a reaction to changing notions of career trajectory and adulthood as feudalism gradually turned into industrial capitalism and

Graeber asserts it's specifically a reaction to changing notions of career trajectory and adulthood as feudalism gradually turned into industrial capitalism and the normal life path, for practically everyone in society, of: 1) work for someone else to learn the ropes and get started (even aristocrats would leave home to serve in other households, unless they were top of the heap), 2) run your own farm/shop/manor/etc was changed to just: 1) work for someone else, selling your time directly, and... that's it, until you retire or die, for almost all people who aren't born rich. I don't have the background and haven't followed his references to dig into how accurate his portrayal is, but his does address this exact attitude in the book. He essentially claims moralizing about the virtues of hard (sold-by-the-hour) labor and long days is a reaction (by the upper classes, largely) against what was perceived to be a growing population of people stuck in perpetual adolescence—eternal apprentices, basically.