Their lifestyle choices put them at risk of diabetes and heart disease. A McDonald's Big Mac is tasty. Eat enough of them, and your arteries will become clogged with fat. It's a risky pleasure. People have used tobacco for hundreds of years.
Tobacco increases concentration, inhibits appetite and provides a stimulant effect.
Additionally, clinical studies have shown that nicotine has antidepressant properties. Finally, for some people, it's cool to smoke. The simple fact is that some people enjoy smoking. Like a lot of other pleasures, people find it hard to stop. That doesn't change the equation.