Human verification is very expensive, compared to unit tests. It costs money to pay that human to do it, time for them to test it, time to describe issues found

Human verification is very expensive, compared to unit tests. It costs money to pay that human to do it, time for them to test it, time to describe issues found, time to send it back for a fix. Unit tests - actually, all automated tests - are comparatively cheap. The developer can run them immediately. All code will have bugs. The "trick" to building a productive development pipeline is to catch as many of those bugs as possible as early as possible, and thereby reduce both the temporal and monetary cost of resolving them.