Great thought. They have great mobility, strength, and power because they train it daily. Our bodies adapt to the stimulus we provide. Many gym goers emphasize endurance and hypertrophy rep schemes, aiming to build muscle and improve race times. If we want to improve strength, we need to lift heavy weight or increase the demand through manipulating the body (rings routine for example). If we want to build mobility, we have to constantly train at the ends of our mobility. If we want to train power, we have to perform explosive movements with max effort.