1) United States agriculture absolutely depends on a certain amount of unskilled labor to function, for which the United States has been importing Mexican laborers in large numbers for at least 80 years. Ending this importation of labor would literally cause the agriculture system in this country to break down.
2) Many parts of Mexico have been economically depressed with high underemployment/unemployment for decades. There are physically too many people for the quantity of available jobs.
The undocumented immigrants I know (including my parents’ godson who is a migrant agricultural laborer, including several guys who stayed in my family’s house for a few months in the early 1990s, including 30-50% of young men from many indigenous communities in southern Mexico, etc.) DO NOT want to leave their parents, their wives, their children, their communities to go to a strange place where they do not speak the language to do hard labor, missing their children’s early years, DO NOT want to take on tremendous personal financial risk in the form of huge loans from loan sharks who will happily repossess their family’s home or start sending goons to beat people up if the money isn’t repaid.
Unfortunately, the choice is often hover-your-whole-life-just-above-or-below-subsistence-level (i.e. take on personally degrading and dangerous jobs to feed your family, risk starvation, etc.), or clear out and go somewhere else. If the outlet of leaving to the United States were unavailable as an escape valve, I’m quite convinced many would end up either starving or turning to other kinds of desperate action.
In other words, not really a choice at all.
You’re damn right that a system that has people staying in this country without papers isn’t fair and allows those people to be taken advantage of. You know what else isn’t fair and allows people to be taken advantage of? The whole global economic order.
[Note: this is not intended as a call to any particular action; just trying to state facts.]