What makes people think that weed does not hurt your lungs?

Any one that smokes weed knows that tar is one of the major after affects from smoking it. How in the world makes people think it not going to hurt there lungs. You know that tar used for building roofs. There is not much difference between the 2. Why do roofers have a 45 year life expectancy. I cant stand some of the stupid answers people use here sometimes. Even common sense will tell you some not right with them.