Why left ventricles thicker and more muscular than right ventricles?

It is a result of thousands of years of evolution. But you also have to look at their functions. The left ventricle is pumping deoxygenated blood through the pulmonary artery to the lungs and back to the left part of the heart (including the left ventricle) which then pumps the oxygenrich blood out in the body. I guess you can imagine that pumping the blood in the body requires more work than pumping the blood to the lungs and back.

I suggest you read more about the circulatory system and the heart if you have an exam or something that you have to prepare to.

Best regards

premed student