Out of all the countries in the world we are pretty close to the bottom when it comes to vacation time and hours per week worked. We are expected to work 40+ hours a week along with running a household that, in most homes, also includes children. We just simply don't have enough hours in the day to get everything done and that usually means taking shortcuts, like prepacked meals. Like fast food. Its much easier to open that box of Mac and cheese than it is to make it from scratch, noodles included. Its a lot easier to whip in the drive through and pick up dinner for the kids than it is to listen to them whine while you're trying to cook and help them with homework.
It then trickles down, those kids grow up and probably don't know how to really cook besides the fact that they grew up eating prepackaged meals and its like second nature to continue eating what mom and dad fed you as a kid.
I think there are many causes of obesity in America, not just overworked adults who don't have time to cook.