And economics. Cars are designed for reasonable loads. Making a vehicle heavy duty for hauling large loads means all the suspension and drivetrain components need to be beefier, which increases weight, which increases cost with the extra materials and engineering that is necessary. When they made the new Miata (my next car!) they worked miracles to shave off grams and ounces from components, even using 4 lug wheels instead of the traditional 5. This means the car cannot carry heavy loads, because then they'd need a bigger engine and drivetrain (more $$ and weight), heftier suspension (more $$ and weight), and tougher frame made of heavier alloys (more $$ and weight).