The bigger reason we don't continually scan people is that everyone has all sorts of little cancerous growths that are entirely benign. They would show up on these scans and freak people out. And often the overreaction of invasively removing those tiny little benign cancers can cause more problems. Don't misuse or abuse this information, but in general the 'abnormal growths' in a person are just that. They do very little and are just little bugs in the system. But the overall system is capable of dealing with them. It's the rare kind that is able to get around the entire body's defenses. But unfortunately it's difficult to tell which/when that happens.
But again, if you took whole body scans all the time, and tried to root out all those little growths, you'd cause a lot more harm than good.