The box shape provides more interior room to hold things, especially if you're spending many days outdoors. The roof is flat so you can put more things on top. Lastly, the shape helps prevent the vehicle from rolling onto the roof like an upside down turtle.

There are downsides however. A box is not aerodynamic so the mileage is poor. At high speeds, the air hitting the box is loud. Mileage is getting regulated by countries for new cars. Comfort is what people are interested in for a daily driving car.

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.)

A few reasons that such vehicles are designed the way they are:

• Legacy/history of vehicles that were all function, like original Jeeps, Land Rover Discoveries that go back 60-70 years

• Aerodynamics don't matter much for vehicles going slow through rough terrain

• straight edges and sharp corners make it easier to navigate terrain without hitting rocks, trees, etc. When you know the edge is then straight down vs. tapering then it's easier to navigate through tricky spots without damaging the body

• Maximizes space inside, flat surfaces to strap gear outside

Off road capable vehicles are like indoor outdoor basketballs, they don’t actually work well outdoors. They’re certainly better than indoor balls (regular cars), but if you want something that’s actually useful in rough terrain, you want something that’s specifically built for it. Not something that claims to do both.