Immigrants do the jobs Americans won't do
Prior to the disastrous immigration act of 1965, there was very little immigration.
In fact, between 1925 and 1965, immigration levels were so low the number of immigrants in the country actually declined; there was even a period of net emigration out of the United States.
Yet, during that time, Americans invented computers, had a healthy labor movement, initiated the space program that put men on the moon, made great strides in civil rights and environmental legislation, built the largest economy the world has ever seen, and successfully prosecuted WWII against two great powers on two fronts simultaneously. We also got our grass cut, and our meat packed. Our children were being watched, and our houses were being cleaned.
The idea that somehow we suddenly can't run a country without an endless supply of foreigners is absurd.
The falsehood repeated endlessly, that immigrants do the jobs Americans won't, is really tantamount to something like this: Imagine the owner of the local McDonald's puts a sign in the window that says: "Dishwasher wanted. $1.00 / hour." Suppose he leaves the sign in the window for a month, but no one comes in to apply for the dishwashing job. "See?" the McDonald's owner might say, "Dishwashing is a job Americans won't do. But there are a billion people in China who work for less than a dollar per hour. I need to import some cheap workers from China (or Bangladesh or Mexico)."
Then he or she will import the worker, undercut American wages, and, as a bonus, stick the taxpayer with the cost of the new worker's health care, of educating his children, and so on.
And politicians will talk about how our economy "depends" on immigrant labor.
A country should do its own work.
Reason #3
We are a nation of immigrants.