Chrisb1 mentioning the "quality of our food reserves" is a nutritional study. The above nutrition book covers foods from A-Z. We crave fatty foods because up until the last 200 years people would starve in times of famine.
During famine your body lives off that stored fat. Supermarkets, mass food processing, canned and refrigerated foods did not exist.
In organic chemistry your body stores that which it cannot break down. Animal byproducts such as eggs, butter, cheese, meat are composed of molecules which have multiple carbon bonds which cannot be fully broken down by our digestive systems. The result is those molecules are stored as fat. Fruit and vegetable products however, can be broken down and as far as I know will not be stored as fat. Starches such as potatoes, white rice, pasta(wheat), are broken down into Sugar or glucose which can be detrimental for those with diabetic gene.
All meats are not created equal. Roast beef is lean and therefore carries less fat. The same can be said of bacon according to the, "Complete Handbook of Nutrition", because if bacon is cooked and patted down with towels after done most of the fat in the bacon grease.