Because when it comes to immigration, the downside to getting it wrong are life-altering.

The expected value for immigrants is rapidly shifting into it being more favorable to be an illegal, because ICE/CBP is mostly going after low hanging fruit of easy to catch people that they know about with homes and salary jobs / university and a visa. People that are off paper and 100 miles past the border are as good as gone. So basically what we get is the exact opposite of what we want.