No, has nothing to do with population density but all to do with legislation, taxation, and advertisement. It started as a regulatory loophole and slowly became a viral advertisement meme.
US, Canada, Australia are the only odd ones out. Even in Mexico most pickup are utilitarian low height flatbeds, same for most Central and South America.
You guys have a large (length and height) vehicle arms race that is absolutely setting back decades of vehicle design, pedestrian safety, and fuel efficiency.