Why is the height of vehicle kept as low as possible?
The height of the vehicles, especially cars, is kept as low as possible to keep the vehicle's center of gravity as close to the ground as possible. The center of gravity is the average location of the weight.
The lower the center of gravity (the closer it is to the ground), the higher the stability of the vehicle. The vehicle has a higher resistance to toppling over if the center of gravity is kept low.
It is for this very reason that sports cars have a very low height, so that they can maneuver sharp corners at high speeds safely. Another safety feature usually seen in those cars is the wider wheel base, so as to get a wider distribution of weight.
Wind shear is another reason. A lowered height gives an aerodynamic stability as well as a gravitational advantage. Driving on a still day at typical highway laws and restrictions, an automobile creates its one wind of whatever the speedometer reads. A boxy delivery vehicle is going to feel that drag, especially at the fuel pump, but a well designed car, not nearly as much.