Why do lungs have no muscle built in them?

Lungs do, in fact, have muscles built into them. The primary muscles involved in the process of breathing are the diaphragm and the intercostal muscles.

The diaphragm is a large, dome-shaped muscle located at the base of the lungs. When it contracts, it pulls the lungs downward, creating negative pressure in the thoracic cavity. This negative pressure draws air into the lungs through the nose or mouth.

The intercostal muscles are located between the ribs. When they contract, they pull the ribs upward and outward, also increasing the volume of the thoracic cavity. This additional volume also helps to draw air into the lungs.