This guy had MRI scans which don't rely on ionising radiation and have no known adverse effects.
>everyone has all sorts of little cancerous growths that are entirely benign
When discussing tumours, cancerous and benign mean different things. You cannot have a benign cancerous growth.
I think the main reason people aren't scanned routinely is due to the enormous cost of the scanning and the data analysis that currently relies on physicians for interpretation. My personal view is that with ever decreasing costs of MRI and improvements in computer vision and diagnostic algorithms, this could become a viable process in the future. As for not over-treating, that is a physician led decision - just because there is an anomaly on the scan does not mean that it will be removed or even necessarily treated at all.