Many areas will have large population centers on the Mexican side of the border, and little to no infrastructure on the American side, making them ideal places for launching points into the U.S.  Some recent efforts have been made to fix this situation. US Authorities have built autonomous border patrol watch towers, Autonomous watch towers scan the environment with radar to detect movement then it orients the camera to that location. I've used something similar to this when I was in Iraq and its detection capabilities are incredible. You can easily spot targets over 2 kilometers away. Computer AI algorithms analyzes the imagery to automatically identify if its worth notifying and sending agents out to the scene. but these are expensive and typically dozens of miles apart. They are only posted where terrain permits at the most easily accessible crossing points.