I have a cousin in Italy who came over a few years ago on a student visa and learned to be a helicopter pilot. During the course of his schooling, he became very interested in becoming a U.S. citizen and be able to stay here. When he looked into it, he was denied. I believe their reasoning was that there was a quota of immigrants allowed from each country and that there was a long (several years) waiting list that he can get on.
I seldom got to see him before his visit and now see him more rarely. I really wished he could have stayed and become a productive contributor to this country like his uncle (my father) did. But the law was the law, and he went back.
Good point. I hear folks say, "the illegals could, do it legally." But in reality they can't.
My husband did it legally, he's Canadian, and was employed by an American company, so he finally got a green card. Then he used his years in residence here (5 is the requirement, I think, ) and he could apply for citizenship.
But if you'll follow the links on this page, and choose Mexico as your nation of origin, you'll see that a Mexican will not be allowed to seek a green card, at least not at this time.
http://www.insexperts.com/cm/getting-green-card.htm