So the reason isn't our evolutionary history, it isn't because of caveman instincts kicking in that say we better eat lots of food because we don't know where our next meal came from. With emerging awareness of neuroplasticity (that the brain rewires itself in response to new information) and the recent study revealing that hunter gatherers actually burn just as many calories as people living more sedentary lifestyles.
What does this leave? Decadence. We look at the world and see how people elsewhere have less things and praise ourselves in comparison for our "achievements" and we celebrate and celebrate and celebrate until we are fat and sick. Shame on us!
That still does not show at all why we like foods that are bad for us. Additionally, it is impossible for us to have measured the amount of calories our far distant evolutionary predecessors burned. Additionally, the theory is not so much based on the idea that hunter-gatherers tend to have more exercise (although this is true, and I'd have a hard time not believing this unless you give some strong evidence), but on the idea that high-calorie foods used to be rare and mostly inaccessible (mostly honey was our only access to "sweets" and the occasional citrus fruit, while most of the fruit we ate was more vegetable-like). Calories help one fight starvation, but our bodies are not evolved to take in so many high concentrations of calories and fats. Decadence is certainly a problem, but evolutionary nutritionists try to look for *why* our natural tendencies are to eat foods that are bad for our bodies. Certainly there is a cultural problem, but we shouldn't deny its history. We need a social evolution revolution.