You are comparing a world power with third-world countries. The only thing I can say in response is, 'lolwut?'.
if you let people in from 3rd world countries with an open border, then we, ourselves, become a de-facto 3rd world country.
The trouble with letting in all the Mexicans in is that then one will be elected President, and he'll let in all the Chinese. Once they start coming, it's all over for Western Civilization. Once they conquer America, they'll move south and the Mexicans will realize they made a big mistake. Last time this happened, the Great Wall of China got built, and you can still see it from space.