your post is not quite true. We dont visually inspect or scan 100% of the cargo. That would be impossible.
Screening includes checking manifests looking for bad signs.
You are both right. I looked again.
"CBP uses intelligence and a risk-based strategy to screen information on 100% of cargo before it is loaded onto vessels destined for the United States. All cargo that is identified as high risk is inspected, either at the foreign port or upon arrival into the U.S. "
You indeed can't physically inspect all cargo. But I also read, that all containers to go past radiation detectors, so they are screened for radiation.