Why do almost all people wait until they get a serious medical problem to then start living healthy?

Hi everybody! Why do almost all people wait until they get a serious medical problem to start living healthy? I'll give you some examples of what I mean in the form of questions. Why does a smoker start stopping to smoke after smoking for 50 years after they find out they got terminal lung cancer, such as emphysema? How come an alcoholic stops drinking after they kill somebody on the road while driving drunk? Why do ually promiscuous people all of a sudden start living healthy after they get a terrible, incurable STD, like HIV/AIDS? Another example could be why do people deal begin to finally deal with their obesity and being overweight after the doctor tells them they now have heart disease or diabetes? People are weird and procrastinate by nature in my opinion. Any answers would be greatly appreciated! Thanks!