Other comments have spoken about why random screening is a bad idea. But the real problem is the tests we have just aren’t good enough. What are you hoping to diagnose with a CT scan? If you don’t smoke, the most common cancer you’ll get is breast/prostate and colon. CT scans are close to useless for detecting those cancers (If you do smoke then there is some evidence for CT screening).

Also note that image based screening has a resolution of around 5-10mm (CT and MRI closer to 5mm, nuclear medicine closer to 10mm). Once a cancer is this size, we are already talking about over 100 million cancer cells in your body. You will need surgery, and possibly chemotherapy and radiotherapy to eliminate these cells once it gets to that size.

What you actually want is prevention, not early detection.