I don't follow? Why would a US citizen want to compete against 8B people for jobs instead of just 300M? Seems like the lower number is better for the citizens of the US.

Would that US citizen be even better off if we got rid of all the other Americans so they had to compete against nobody?

Immigrants contribute to both supply and demand. It's not at all obvious whether this is net good or bad for natives' pay.

Why not just decide that only white people or only men or only people born on Tuesdays can apply for the job you want?

The job should go to the most qualified person. That's how america works. If that most qualified person is actually a guy who was born in india, great. I have no problem with that. You don't get to bogart opportunity based one what zip code you were born in.

What is the purpose of a nation if not to benefit its own Citizens. If it doesn't do that in good times then why would anyone fight when called upon?