Good luck collecting that..
Mexico has much to gain by the status quo. Millions of their poor and unemployed people head into the U.S., legally or not, and not only make livings for themselves instead of collecting some form of Mexican welfare, they actually send billions of U.S. dollars back into Mexico.
If our government had a welfare system that not only exported the problem population, but actually MADE money for the U.S. they'd like it, too.