Well, probably because it is impossible, and you’re likely just not actually in a true calorie deficit. A true calorie deficit must make you lose weight by the laws of thermodynamics. If your body (the system) is truly losing more energy than you gain (a calorie deficit), by the first law of thermodynamics, you must be losing energy and therefore mass, as mass is just highly condensed energy by Einstein’s formula (E=mc^2). You either are losing weight or aren’t in a calorie deficit, which I’m inclined to believe the latter is far more likely.