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