Because GP's reasoning is likely not to be correct. The proliferation of cheap, high calorie foods along with a sedentary lifestyle (including occupations that require very little physical activity) are more likely to blame than 'cars did it.'
I would add that food corporations devote a lot of resources to make people eat their junk. It's not only that the food is there to eat. Companies like Coca Cola or Mac Donald's spend billions to convince people to eat their stuff.
