CT scans expose you to about 100-1000x the radiation of a single x-ray but that isn't the main reason frequent tests aren't done. The main reason based on my understanding is two-fold: First, cost would be prohibitive, and second the absolute number of false positive would do more harm than good overall.
I wonder if the relative rate and effect of false alarms is constant or if it increases as a function of how frequently scans are done. This is related to what studies are finding about cancer screenings. The more screenings you have, the time at which potential tumors are spotted is pushed back, and more non-dangerous tumors are caught. These benign tumors are mistakenly treated as malignant tumors because they were detected too early. I would imagine something similar would happen if people went for all out CT-scans.
