Why do athletes have developed not only the muscles of the limbs, but also the heart and chest?

Athletes train not only muscle strength, but also endurance (for example, when running, skiing, etc.). Such loads are called cardiovascular exercises. During them, the heart is actively working, which is a muscle, and accordingly also trains. In addition, cells require more oxygen, so breathing becomes more active. This is ensured by the work of the chest, because the lungs themselves are not muscles and cannot contract and unclench on their own.
With cardio training, the heart becomes more resilient and the lung volume can increase.