Because the industrialization of America is over, and has been for decades. USA doesn't need low-wage, immigrant workers anymore. The railroads have already been built, the fields have been plowed, and now that's all done by big automated machines. Everything that cheap workers used to do that was valuable is now automated.
You're putting the cart in front of the horse. If the US economy didn't need low-wage immigrant workers, we wouldn't be complaining about them in the first place, because they would've gone somewhere else where the jobs are.
The fact they're coming the US literally means its economy needs them.
Of course we could all wax philosophical and say "Nobody needs a Frappuccino every other day, we just want it," but then nobody needs to live in a prosperous economy anyway.
I wish i could ill find the video, the farms in CA certainly do need labor. In the 90s when there was another - people south of the border are taking jobs bs - an interview er asked people waiting for for support at a welfare office in Salinas (lots of farms) offering jobs in the fields. Unanimous nope.
They are needed and often do more than those that are citizens.
Who does the farming? Who does the cleaning? Who builds the buildings? Who are the line cooks? That should be obvious.
But it should be just as obvious that there are plenty of immigrants who are also necessary because they bring new ideas, their education, their incredible work ethic, to fill in the gaps that the US clearly has.
There is one thing that unites all of us (and I do mean us, as I am one of them). We all dream of a society where our hard work can become prosperity for ourselves and for everyone else, a plot of fertile soil that is worth sowing. We all come here with a dream.
And I personally don't mind so much that I'm uplifting people that don't agree with my existence. I just wish that they could stay out of our way so we could all benefit.