Or perhaps people just have different values than you? Perhaps they are fully aware of the cancer risk and have full agency over their decisions yet still prefer to indulge themselves?
Quitting smoking is one of the most difficult things for someone to do. Some people can go cold turkey and power their way through it. However most people struggle because of their body's biochemistry. When discussing this topic I've noticed common responses along the lines of, 'that person didn't actually want to quit', or 'that person didn't want to quit strongly enough'. Yet I see it as their limited agency was unable to overcome their body's physical dependency.