Think of the Wall as a Gun Free Zone in the states...
Would the person who wants to fuck things up with a gun follow the rules of a gun free zone? No, they wouldn't...
The Wall is like a Gun Free Zone, it's just a facade by a Con-man wanting to provide a false sense of security with something that isn't much of an issue.
The percentage of illegals from Mexico itself according to Pew statistics is lower than it was in 2007. It makes no sense to put something up that is proven not to work effectively.
The focus shouldn't just be on Mexico being the scapegoat, there are other nationalities that find ways of getting into the states...