Why do so many people oppose to immigrants?

They're taking our jobs! Yes I've heard it all before but the fact is that they DON'T take our jobs. They take the jobs we don't want like they become janitors and get paid really poorly and send the little money they make to their families to help them out. So why do so many people not want immigration? All they're trying to do is make life better for their children and what's with this wall America's trying to build to keep Mexicans out? Let me get this right. You want the Mexicans to build a wall to keep out themselves from the country? I don't get why do many people have a problem with immigrants. Can someone please explain this issue to me?