Why must resistance training be paired with increased calorie intake to gain lean body mass?

It signals to the body that extra calories should be directed toward muscle repair and growth

Gaining lean muscle tissue, distinct from simply gaining weight (which can easily be stored as fat), requires a specific biological instruction. While consuming a calorie surplus provides the raw materials (amino acids, energy), resistance training acts as the crucial signal. When muscles are subjected to the stress of lifting weights, machines, or body weight exercises, microscopic tears occur in the muscle fibers. The body then utilizes the abundant incoming calories and protein to repair these tears, leading to hypertrophy, or growth. Without this stimulus, the surplus calories are far more likely to be directed toward adipose (fat) storage, defeating the goal of increasing lean mass.