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.'

Suburbs are growing farther and farther away.

More indoor entertainment than ever (Internet, video games)

No free spaces, parking lots for kids and young adults to just go out to walk/play

Took some time for the cumulative effect of all this to show up.

If you look at the obesity charts, year by year, you can see precisely the year obestity took off like a rocket. It was the year following the US Gov announcement that Fat is evil. This caused all the food manufacturers to start pumping everything full of sugar. In the coming decades, gym memberships increased quickly to no avail - you can't out excercise a terrible diet.

The rates have been steadily going up since that time, though. It makes sense that you would see a gradual ramp up in a drastic change like ths.

I don't have sources to back this up, but here's some thoughts:

* Gradual changes in types of labor? (From agriculture -> industrial -> office jobs) which are less calorically intensive.

* Changes in food? Calorie-dense food is cheaper and easier to get than ever. People have less free time to cook healthy meals (working more hours, far fewer homemakers, longer commutes, more time spent doin extracurriculars outside the home, etc). As a result, people consume more calories because they're easy to get.

* Health/Food misinformation. The demonization of nutritional 'fat' which lead to it being replaced by sugar, which is even more addictive.

* Changes in drink consumption? Alcohol consumption is up (at least compared to ~25 years ago), and alcoholic drinks area very high in calories.

* Childhood changes due to changes in food and care. More high-calorie, "manufactured" foods introduced earlier into peoples lives sets them up for a lifetime of bad habits and problems that only compound over time.

Add in addictive computers, TV, and vidya - and that's probably your answer.