Why are cars made with the ability to go over 100 mph when the maximum speed limit is only ever 70 mph?

would it not save more lives to just make the car with a top speed of 70 mph to stop so many road deaths due to speeding i know public services such as police fire and ambulance should have there cars made as fast as possible the same for race car/bike drivers but would it not just be common sense to make them with...