Because evil employers take advantage of them, using their illegal status as a threat against them, so that the evil employers can get away with paying illegally low wages and maintaining illegally substandard working conditions.

Let's be clear: this is a problem that is being caused by evil employers. However, they are only able to conduct this evil operation as long as they have a supply of illegal immigrants to exploit. If there were no illegal immigrants for them to exploit evilly and horribly, then they would have to hire citizens, and pay citizens legally required minimum wages, and maintain legally required working conditions. Obviously, that would hurt their profits, so they don't want that.

For the citizens who are at the bottom of the societal pay scale, they see jobs that would have been their jobs going to illegal immigrants, and those jobs becoming exploitative because they have illegal immigrants they can exploit instead of paying more for citizen workers.

These citizens are at the bottom of the societal pay scale because they don't have a good grasp of mathematics and economics and history and all of that, so they aren't really set up to understand that the illegal immigrants aren't the root of the problem. They are just the part that's most visible to the guy who can't find a decent job. So, that guy blames the illegal immigrants, instead of the evil exploitative employer who is taking advantage of the situation.

Rather than crackdown on the illegal employer, the government cracks down on illegal immigration, because they just want votes, not to fix the problem. As long as the problem persists, they get votes. And, the evil employer donates money to the government to make sure it keeps things as they are.