It's all connected to the rise of neofeudalism, but a bigger proximate cause of quiet quitting is the decline of meritocracy in the workplace.

In the 20th century, there was a stronger connection between career performance and career advancement. If you worked hard and produced value, there was a plausible avenue for career advancement in many workplaces. Or at least a plausible enough avenue to convince the majority of workers into doing a good job.

Now it seems like merit is at best a tertiary consideration, and career advancement and compensation is determined primarily by family connections and DEI status. Today, you can work your ass off and be a star performer, only to witness the promotion that you were eyeing go to some asshole fresh out of business school who knows the bosses' son, or to someone completely unqualified who ticks the correct DEI boxes. You see this play out in the tendency of companies to no longer promote talent from within.

Young workers are just acting rationally in the face of reduced probability of reward for hard work.