'Cuz your dad (along with me and a few others past our prime) grew up when medical XRays were frequent and at far higher dosages than today's, as children were taken by our parents to be fitted at shoe stores with "Shoe-Scope" fluoroscope XRay machines, and by "current" standards were "radiated" far more than the modern "sensitive" folks would tolerate. Of course, many of us were also "OD"ed on pencillinn and the second generation miracle antibiotics for non-serious and often wrongfully treated illnesses, making us the vectors by which all sorts of infectious agents could mutate to become resistant. Then we ran down the street in the tail plumes of the regular mosquito spraying trucks, dined on fruits and veggies laden with insecticides, and even if not in Viet Nam sure absorbed a spectrum of herbicides near as carcinogenic and dangerous as Agent Orange.
But then, I can recall seriously and liberally "dusting" a small Italian orphanage and its tenants with DDT. In retrospect, it wasn't wise, but then neither was living in a tiny ship's stateroom through which a large lagged steam pipe passed. of course, the "lagging" was packed with asbestos fibers, thumped by every catapult shot into the air I breathed. If I'm going to worry, I guess I'll worry about the risk factor from that exposure. I forgot all the dust from 3 years of life with constant exposure to the dust from lead paint, a practice which on old ships was constant as long as it was light enough to see.
Maybe, at his age and my age, we're simply not too worried that a little more radiation by the TSA is likely to be a threat to our health. For your dad and me (and others), there ain't no do-overs.