Why the walls of atrium are thin?

only about atrium thinner not than ventricles plz​

This is because blood is pumped out of the heart at greater pressure from these chambers compared to the atria. ... This is due to the higher forces needed to pump blood through the systemic circuit (around the body) compared to the pulmonary circuit.

The walls of the atria are thinner because the atria doesn't need to pump blood to the whole body, it only needs to transport it to the ventricles.