Because "undocumented immigrants" is just another term for "illegal aliens." These people are actually breaking the laws of the country by living here without becoming citizens or possessing green cards, work visas, etc. Because they aren t going through the legal process to stay here, they don t have to pay taxes, which means they re working jobs and living in homes/apartments/camps/wherever without having to pay anything to the government. In conclusion, it s a crime, they are criminals, but they go unpunished. It s not a hatred of them, it s just the fact that they should follow the laws of the country, or face consequences, just like any other person should have to do.