The U.S.-Mexican border is essentially wide open.
Why? Because there is a general expectation in Mexico and Latin America that American immigration law is unenforced. Or it is so bizarre that simple illegal entry almost always ensures temporary legal residence, pending an asylum hearing.
A scheduled asylum hearing, in turn, is seen by border crossers as a mere formality to be ignored. The popular perception on the border, then, is to stick one foot illegally onto U.S. soil, and, presto, win permanent residence for you and any family members who wish to follow.