Why not? Because of the well known, widely understood problem of iatrogenic harms. Your conclusion does not follow from your premise. If you have a predisposition to a particular disease, get screened FOR THAT specific thing. Getting a full body scan just opens you up to unnecessary psychological distress, unnecessary treatments, and unnecessary healthcare costs for many things which would have never affected you in the slightest had you avoided getting a full body scan. Do you REALLY want to know that you have an aneurysm that only has a 1/100 chance of rupturing, when you would've otherwise gone your entire life without ever worrying about whether tonight will be the night that it ruptures and you won't wake up tomorrow? Do you really want to know that you have a slow growing cancer that would only affect you when you're already less 12 months from your ultimate death at age 92, but instead you got invasive life-altering surgery at age 65 because it just happened to show up on a scan? If we had instant, easy cures for every disease, then yes, get a full body scan - unfortunately this just isn't the case, and the harms greatly outweigh the benefits.