Why don't Illegal Aliens (aka "undocumented workers") make like a tree and leave?

I walked through an area of town they hang out in today...there were empty Modelo cans on the ground, garbage everywhere, and I saw a bunch of them getting off the bus....then went a few blocks away and the area where they don't hang out is immaculate. I've been to Mexico when it was only violent, before it became suicide to enter...and it looked the same. I think its similar.