Explain what nobody on your side has yet to explain
How do police stop crime?
How do locks stop break ins?
How do laws stop violence?
How do vaccines, good diets and exercise stop disease and death?
How do prisons stop escapes?
The answer is simple. Walls are but a first line defense like everything else to discourage and make unlawful activity HARDER. Nothing is absolute, nothing CLAIMS to be absolute, but combined with good technology, intelligence, and port management, they collectively cut way down on what the crime might be otherwise.
And for every billion we spend on the wall, that should net roughly a SEVEN billion dollar return on the resulting reduced costs related to handling and processing of the illegals that otherwise would have come in there. It really is a very simple decision. Even if we are somehow 700% off on our calculation, it is still a zero sum gain. We break even and the wall didn't actually cost us anything in the grand picture.