Sure. So theoretically you can accomplish calories in < calories out by eating nothing but sugar. In practice, this doesn't work very well because sugar a) isn't filling; and b) trains your body to crave more sugar; and c) eating nothing but will leave you with deficits in pretty much every nutrient your body needs to thrive, which will in turn cause cravings. Nobody I've ever met has enough willpower to sustain a diet under those conditions.

The difficulty of weight loss isn't the math. It's how to hack your mind and body to minimize the willpower needed to maintain a caloric deficit that millions of years of evolution have optimized your system to avoid.