There are quite a few reasons why modern cars are the shape they are, the wind tunnel is one of the main reasons why most cars look similar, then because the cars are made of thinner metal the panels have contours pressed into them to give strength and structure, add in crumple zones and safety features like side impact bars and the overall size can increase without any increase in the interior. High end cars with carbon fibre can get round this, but is expensive, take the engine out and add electric motors and batteries and then the shape can evolve a bit, but fuel consumption is god, so the wind tunnel still dictates the shape.