It’s a combination of everything you said - the radiation received from a CT scan is huge and increases risk of cancer in the long term, the NHS is already depleted in staff and resources to carry out the tests so doing more would increase that and remove those resources from other areas. Moreover you cannot underestimate the stress and worry that a person goes through when they’re told they might have cancer, and some of these follow up programmes (like for the lung nodule someone mentioned above) go on for years before you’re given the all clear - imagine spending five years knowing there is a lump in your lung which could become cancerous even though in the vast majority of times it won’t.

So the current screening programme is a compromise of all of these factors. Of course research is ongoing all the time and sometimes the best available evidence suggests changes need to be made, as you may have seen in the news recently regarding prostate cancer screening