I'm not a smoker, but there are many reasons why people smoke. Until these are truly known and, whenever possible resolved, it is almost impossible to stop smoking. In a crisis, such as the one we're experiencing now, many people reach for some support. In some, it is smoking, in others drink, or in others even even sex. Giving up any of them seems impossible for them.
As for killing themselves - other people do all sorts of things, which may be considered dangerous e.g. driving a car at high speed, bungie jumping. Some have impact on others as for example sitting behind a wheel when you're drunk may result in an accident and innocent people being killed.