Food companies putting more sugars, salts, and stuff to make the food more tasty while advertising it as healthy because it has less fat. Plus we have TVs, computers, so people aren't moving as much.
I am not saying Twinkies, McDonalds, Doritos, and Coca-Cola are bad for you, but consuming those things in large quantities while doing nothing is bad for you. At the same time fat is not entirely bad, but sugar can be.