Often with insufficient muscle mass, they talk about a protein deficiency in the human diet. Why not talk about carbohydrate or fat deficiency?

1) This is due to the fact that proteins in the human cell are synthesized from amino acids, which are formed during the breakdown of food proteins during digestion. Essential amino acids required for protein biosynthesis cannot be synthesized from fats and carbohydrates.
2) Carbohydrates and fats can be synthesized from proteins supplied with food, therefore, their deficiency is not observed.