Why do the circulatory systems of land vertebrates have separate circuits to the lungs and to the rest of the body?

Because it prevents venous blood from mixing with arterial blood (which is rich in oxygen), in this way the circulation is more efficient.

Explanation:

In land vertebrates, the blood circulation is structured in two independent circuits: the pulmonary circulation, where oxygenation of the blood occurs and the elimination of the carbonic anhydride that it contains, returning back to the heart through its left atrium; and the systemic or major circulation, impelled from the left ventricle, transports the oxygenated blood and the nutrients that it assimilates as it passes through the digestive system, to the tissues of the animal, where it is charged again with anhydride carbonic and other waste substances, returning back to the heart, where it enters through the right atrium. These systems are independent and prevents venous blood (which is poor in oxygen) from mixing with arterial blood. These systems are independent and prevents venous blood (which is poor in oxygen) from mixing with arterial blood.