It's an oversimplification of how the body works to the point of not being useful. For example, if you eat 2500 kcal a day and maintain a steady weight of 150 lbs, it is not the case that if you change your diet to 2200 kcal you will consistently lose X pounds per week. You would more likely lose a bit, then plateau at some level that's hard to predict, because now your body adapted to an input of 2200 kcal a day. Add to this the complication that where those calories come from matters a lot, because when you increase your blood sugar, your body increases insulin which builds up body fat, but if you have eat a low-sugar/carb diet, that happens less. And if you eat sugary foods, you will tend to get hungry more quickly than eating protein-based foods. It's all so person-dependent and food-dependent that just saying "eat less calories and you'll lose weight" does not accurately describe most people's reality.