Offroad vehicles are typically designed to go slowly over rough terrain. Their shape does not matter for aerodynamics, and being boxy means the most storage space for any given footprint.

At the opposite end of the spectrum are sports cars and cars designed for efficiency, such as the Chevrolet Corvette and the Toyota Prius. Those still have to have the space they need to do their jobs (space for two people plus a large engine for the former, space for people and cargo for the latter) while still being good aerodynamically.

Then, you have white vans, lorries/tractor-trailer trucks, box trucks, etc. Those are designed for the road, but need to maximize cargo space. The fronts tend to have as much aero design as they can get away with, and the sides often have skirts, etc. There is a limit to how efficient the design can be, but if you can carry enough stuff in the back, the efficiency matters less.

Back to offroad vehicles, that is why they were originally boxy. Nobody cares what shape offroad vehicles, bulldozers, or forklifts, were. Over time though, those offroad vehicles became the luxury vehicles as well, and space inside luxury vehicles matter. So, even while it is possible to try to make them less boxy, people have long associated "boxy" with "good offroad", which draws people to buy these box-vehicles, even when they will never take them further offroad than parking on the grass at the park. (On a side note, this has caused people who ACTUALLY need large offroad vehicles to be able to get them used more cheaply than before, and often in better shape because these "road queens" never left the road.)