It can be hard to predict how aggressive a tumor will be. It isn't uncommon for people to obtain cancers that they "die with" instead of "die of". In those case

It can be hard to predict how aggressive a tumor will be. It isn't uncommon for people to obtain cancers that they "die with" instead of "die of". In those cases, treatment can be worse for the patient than the cancer. But since you don't know which it'll be, people get the treatment and sometimes wind up worse off than they otherwise would have. And as the GP post cites, the scans themselves can cause health problems down the road. Performing them on massive scale will increase the occurrences of those side effects.