You speak the truth... but like many things, it's not quite that simple. 

Our food system isn't designed to deliver healthy, nutritious food. It is designed to deliver cheep, high calorie, high-fat, high salt foods that are easy to manufacture, store, cook, are tasty, and satisfy hunger. Notice we are missing healthy in that list. In some places, it can be practically impossible to buy fresh fruits, veggies, lean meats, and healthy fats. In almost every case, these things are way more expensive than the mass-produced garbage. 

Our cities are designed to favor automobiles at the expense of active means of transportation. The mere mention of adequate sidewalks or bike lanes, sends a lot of people into near rage. There are no more shops or business in neighborhoods. Instead, suburban mini-mansions surrounded by distant box stores has become the norm. 

A lot of other factors at play here, but I think we need to do more than just say "go for a walk," and "eat better."