Because you can no longer trust politicians to adhere to laws they would make, not to mention the ones already made. What's keeping your comprehension from understanding what sanctuary cities are? It's a place to ignore immigration laws in Nancy Pelosi's city she represents. More laws will not curb illegal activity. You understand the emergency? It's like making more gun laws when criminals ignore the ones already on the books.Then why build a wall, why not build a working expedited legal immigration system?
A wall can't be ignored. Is it the right solution, probably not but it will help and can't be ignored.