Because your body uses fat reserves really carefully. All your body does is notice you're not getting any food and so it assumes there is currently a food scarcity. Your body will then start slowly burning your fat reserves so you still get some caloric fuel, but not as much as you would normally get when eating. This is because fat in your body is treated as a reserve, and therefore your body will ration it to prevent you from burning it too fast. This happens no matter how much fat you have stored in your body. Because of this, you risk burning too little calories and that can result in other problems.

And even if you endure all that without any problems and reach your ideal weight, your body will immediately start rapidly replenishing your fat reserves for when another food scarcity occurs. This will result in you getting a part of your weight back very quickly. And while your body is gaining that weight back, your daily available calories are still limited because your body needs to store that energy.