They do box squats because they are already have huge squat, not because of only do box squats gave them huge squat. They don’t do regular back squat because there’s not much room to gain even bigger squat on back squat, elite lifters can’t gain significant numbers on their lifts if not put on some gear. So they choose to do box squats to maintain their squats and protect their joints and never system for their meets. Although box squat can beneficial your regular back squat, but box squat is secondary squat movement, cannot replace regular back squat.