It's pretty simple, they don’t.
Too many calories in vs out causes fat gain! There is your answer!
Overeating protein or fats will similarly cause fat gain if you exceed your caloric needs for that given day.
Fats and protein are essential for survival, but carbohydrates are the body’s primary energy source and must not be omitted from the diet. We need protein and fats, but for optimal workouts and repairing/ improving lean muscle we absolutely need carbohydrates too.
Cutting carbohydrates simply cuts calories - reducing fats to a minimum would also cause fat loss.
Eating the right amount of carbs will result in more consistent energy levels throughout the day, improved mental and physical performance and a more sustainable lifestyle on the whole.