It seems that you have never encountered outbound immigration control at the land crossings. They do exist. They are temporary now, and there are no dedicated facilities to perform them, but they are very real. CBP just stops all outbound cars (whether US citizens or not) and checks their immigration documents. I saw them redirecting several outbound cars for secondary inspection. So, your "there will never be exit controls on US citizens" is non-sense because the exit control already exists.