Those of you who are going on about "make them fix Mexico" etc. instead of coming here are right on the money.
And that's the reason the .govs on BOTH sides of the border are not serious about making more than pro-forma displays of border control.
Mexico practices "win-lose" capitalism. They believe they need their poor underclass to stay where it is for their society to function. And there are institutional barriers to individual economic improvement. And Mexico likes to point fingers at the U.S. for slavery, and the history of Black/White problems here, but Mexico is a very divided nation, the darker you are, and the more Native blood you have, the lower you are, while the lighter and more Spaniard you are, the higher. And of course, add to that narco-terror, and massive corruption/kleptocracy, and what Mexico looks like is not hard to understand.
By comparison, the U.S. at least tries to practice win-win capitalism. We believe if a poor person makes good, it's great, they're no longer a burden on society, and they generate even more economic activity which creates opportunity and business for even more people. We may have barriers to this in the form of the welfare state, and "progressive" tax policy, but it's not an American cultural value that the "poor should stay where they are". As a rule, we all wish they WOULD get ahead.
So in Mexico, anyone with the gumption to "do something about it" comes here to the U.S. instead of demanding reform, or causing revolution. This is a HUGE pressure-relief valve for Mexico to maintain the status-quo, and of course their .gov knows this, to the point they openly criticize any enforcement of the U.S. border, and even provides maps and safety advice to the people trying to leave.
IMO, the powers that be here in the U.S. put up with it because they fear that a real crackdown on the illegals would raise the pressure-cooker effect in Mexico, and what would result might be very much NOT to their liking, and look something more like Venezuela or Cuba.
Meh... I'd take the chance.