Why do endurance athletes load up on carbohydrates before a long event?

Answer: It increases stored muscle fuel for sustained exercise

It adds new muscle tissue within a single day — A rapid increase in body mass after carbohydrate loading is not rapid muscle growth. Stored glycogen is accompanied by water, so the scale can rise without new muscle tissue.

It replaces the need for training before competition — Extra fuel does not replace cardiovascular and muscular adaptations from training. Carbohydrate loading prepares energy stores, while training builds the capacity to use them during competition.

It increases stored muscle fuel for sustained exercise ✓ — Extra carbohydrate can increase muscle glycogen stores before prolonged exercise. Those stores help sustain harder efforts and delay fuel-related fatigue; carbohydrate loading is mainly useful for long events, not a guarantee against exhaustion.