It's "get tough" politics. Unions donate billions to Democrats, and they are unhappy with immigration in many industries. (ie. Meat packing, whose union membership has declined dramatically as a result of companies being allowed to exploit undocumented labor.) On the other side, you have conservatives, who are always up in arms about the immigration "crisis". These folks are balanced by the business-focused republicans, who need immigrants to make money.

The end result? Instead of allowing legitimized, orderly immigration from Mexico and Central America, we pretend that that it isn't happening, and then spend billions on needless social services and draconian law enforcement.

Both sets of my grandparents were immigrants to the United States from Ireland. On my mother's side, my grandfather had a 7th grade education (common in rural Ireland) and my grandmother was pregnant when they immigrated here after WW2. All they needed to do was scrape together a few buck and fill out some forms, and they arrived in New York as legitimate resident aliens, able to get jobs and function fully in society.

Today, my grandparents would be accused of moving here to give birth to an "anchor baby" and would probably be here illegally, unless they won one of the elusive green card lottery slots.