Why do ventricles have thicker muscular valves than atrium?

The walls of the ventricles of the heart aren't thinner than the atria, they are thicker.

This is because they require more power as they pump blood around the body, while the atria only pump blood the short distance into the ventricles. More muscular walls are therefore needed to provide this power which is why they are thicker.

The left ventricle is even thicker than the right ventricle as it requires a lot more power to pump blood all around the body. The right ventricle only pumps blood to the lungs.