Why do less active americans not increase their activity levels?

I believe it because "There are a number of factors that influence the sedentary lifestyles of many Americans, but the most simple explanation for this is the convenience with which we are now able to live our lives in the 21st century. There is also a culture that praises working constantly with little free time left over for exercise, which means many Americans are simply too pressed for time or fatigued to become more active."