Calories in -> calories out is flawed (or, rather, not useful) because metabolism is a feedback loop, not a one-way serial process. The types of foods you eat, how they're prepared, and when you eat them have complex influence for how hungry you feel and how much energy you have to exercise or resist impulses, as well as ramifications for the state of your physiology, per nutrient intake.

CICO helps explain weight management issues retrospectively, but it's inadequate with regard to planning, and for maintaining quality of life while working towards a weight management goal.