Legitimate question

I never understood why Americans use the argument "The immigrants are stealing our jobs!!"
What I don't understand is; why are you worried about someone stealing your job in the first place? If you're doing a good job at what you do, nobody should be able to take that from you. And if this immigrant has the better skill set and higher knowledge of YOUR job to the point where an employer would choose that immigrant over you, isn't that just how the job industry works?? If this immigrant has, what I would assume, less resources in the field and STILL does a better job than you, to the point where you are replaced, how is that not fair??
Also, which jobs are immigrants stealing exactly? Aren't they typically taking the jobs Americans won't do? (Farming, drivers, maintenance workers etc...)
I don't know, really never understood the "they're stealing our jobs" argument. Can anyone enlighten me?