If only this worked on the southern border

So it truly amazes me how on the one hand we can have Northcom and a senior commander specifically for the defense of United States territory (General Victor Renuart Jr.) and still have essentially an open southern border (not that the northern border is much better) and out of control immigration. We hear today that our missile defenses are finally nearing prime time and could even be deployed today if an emergency was to develop. If we can do that, why not be able to erect something as simple as a fence on the southern border. Maybe include some proximity sensors and claymores for added effect?