Jus a question. If you have alot of fat. And you lift alot. And eat enough protein, while on a slight deficit. Why doesn't your body tap into that fat as an energy source to build muscle, rather than dietary calories, because, after all, it's dead weight and there's so much of it?
In addition to what kyoun1e said,
I'm going to go layman's terms: it takes more energy input to release stored energy than to utilize what is already floating around from one's food intake. As usual, one's body prefers to take the path of least resistance.
By going into a caloric deficit, one is 'convincing' their body to pull energy from the fat stores.
Going full circle: For most cases, losing fat requires one to convince their body that it needs to utilize the energy in the fat stores. By going into a sustained caloric deficit, one is 'telling' their body that there isn't any food around, and to fill the deficit with energy from fat stores [your body needs 2500 cals (maintenance) and one is consistently taking in 2000 (deficit intake), one's mitochondria needs to make up that 500 calorie deficit by pulling energy out of fat stores.
In order for that to be done, one's body needs to up or downregulate plenty of hormones and generally shuffle around a lot of one's biochemistry.
This 'shuffling' is opposite of the 'shuffling' needed to build muscle. Muscle tissue is not 'designed' to store proteins. It is simply a contractile unit. Taking in a ton of energy alone will not make one's muscles grow. The protein does provide the 'building blocks' and the other macronutrients do provide the 'elbow grease' to build up the muscle. However, because muscle is not designed as a storage unit, they respond to work in order to grow.
When you apply work to your muscles, they can either adapt to the work (get stronger and/or bigger --- this means increasing muscle mass) or acquiesce to the work and simply not grow. At surplus and proper work, they will grow.
The most common reason why the latter occurs is that the individual is not taking in enough calories. Even eating at true maintenance (calories in = out, this is nearly impossible to do, and this is why people recompose slowly over time), one does not have the 'bricks(protein surplus/positive nitrogen balance) and mortar (caloric surplus) in order to build a house (muscle). Bodybuilding and physics are overlapping magisteria, and physics says that you cannot build something out of nothing.
When one is eating at deficit, not only does one not have enough energy to sustain their own mitochondrial needs at, but now the body is required to defend against the work that it is being exposed to. As mentioned, one option is to grow bigger and stronger, however, there is no materials available to do so, the muscles will not grow bigger and stronger.
These concepts are why it is recommended to lift with low volume during a cut. One is trying to apply enough pressure to the muscles to let one's body know that they are needed without forcing one's body to surrender to the work applied.
All in all, bodybuilding is a game of thermodynamics -- fat loss responds to taking in less energy than required. Muscle gain responds to work, however surplus calories are needed in order to build muscle (thermodynamics, again).
As stated earlier, one cannot be in deficit and in surplus simultaneously.