Because your metabolism changes. If you ate 700 calories per day it would be harder for you to lose weight than if you ate 1400 and worked out.

You’d still be eating less, but retaining more weight. Why? Because your metabolism slows down significantly if you don’t fuel it, and that makes it harder to lose weight, not easier.

Most things in life are not binary, and are affected by more than just a single choice.

How do people not realize that the digestive system is dynamic and that as you change the calories in (and the type of calories) that the body changes the out.

You can't outrun thermodynamics. But your body will change its efficiency.