There are reasons why this is probably not a good idea.

1. Even the best tests have a chance that the scan will produce a false positive. If we indiscriminately test everyone for every disease under the sun, most of the findings will be false positives.

2. The benefit must outweigh the cost of the test. Both in terms of expense, and intrinsic factors such as radiation exposure.

3. Certain things that show up in a scan would never cause a problem if they were just left alone. In these cases detecting it early has little benefit, and may lead to unnecessary interventions.

4. If a cure is not available, testing for the disease may not always be appropriate, from an ethics point of view.