A couple of reasons:
surfactant - a detergent - reduces surface tension to a negative value.
the plura contains a "vacume" (lower pressure than atmospheric) and pulls the lungs to the chest wall.
venous blood partial gas pressure is sub atmospheric.
The pleural cavities, ventral and thoracic
The thoracic cavity is the anatomical region with the lungs being situated inside the right and left pleural cavities that flank the pericardial cavity .
Normal expiration is brought on by the relaxation of the diaphragm and external intercostal muscles, causing the thoracic cavity to decrease in volume, which increases the pressure in the lungs. This increased pressure pushes air out of the lungs.
contraction of the diaphragm and the external intercostal muscles, causing the ribcage to expand and the lungs to fill with air. This process creates negative pressure in the thoracic cavity, drawing air into the lungs to facilitate respiration.
Thoracic cavity
The force responsible for normal expiration is passive recoil of the lungs and chest wall. As the diaphragm and external intercostal muscles relax, the elastic recoil of these structures causes the lungs to decrease in volume, leading to expiration.
Heart and lungs.
Thoracic cavity
no
Thoracic cavity
The lungs are located in the thoracic cavity.
reduces intra-thoracic pressure which creates suction that draws air into lungs