They came for economic reasons, and because they had family in California. My grandfather was a photographer; my great-uncle Tom, his brother, had a chicken farm in the San Jose area. They lived in San Jose for awhile, then moved to Vallejo around 1930, where my grandfather opened a photography studio.
When they arrived in California they had 4 children: a 19 year old daughter, a 15 year old son, a 9 year old daughter (my mother), and a 2 year old son. Here is the course of their lives:
- The 19 year old daughter contracted TB from a carrier in her nursing class, and died in November 1928.
- The 15 year old son attended Stanford University, married a young woman from the Napa Valley, and eventually got a job as a manager in a firm manufacturing aluminum windows. He had 3 children, and died in 1989.
- The 9 year old daughter eventually attended college briefly, dropped out to work to help support her family after her father died, and very eventually (1944) married a man from Missouri, whom she met at the USO in Vallejo (my father). She had 2 children and died in 2000.
- The 2 year old son contracted TB in his hip bone, probably from his sister, which left him with one leg shorter than the other. I remember him using an orthopedic shoe built up to near 6 inches. He attended U.C. Berkeley and other universities, ending up with a Ph.D. in Economics at Michigan State University. He married a woman from New York, had one child who died at birth, adopted 2 other children, and died in 1995.
Many people who emigrate to the United States today look pretty much like my grandparents, except for their skin color and language. Things weren't so great in the old country, they hope to make a better life here, the children they bring (or produce here) go on to become Americans, and produce more Americans.
Americans could be rough on immigrants. We've always had a xenophobic streak, not necessarily aimed only at people of color. The Irish, and later Italians, were unwelcome because they were Catholic, an early echo of the anti-Muslim prejudice we see. Now, of course, we also have war refugees, many but not all Muslim, simply running for their lives from countries like Syria, Yemen, and Somalia. My reading suggests that in the early 20th century, Americans often disliked immigrants, because they weren't "like us", but they weren't afraid of them, as we're now being urged to fear Muslims.
Most immigrants just want to make a better life here. Now there's a very small group of people - a few thousand out of tens of millions - who believe that killing infidels (including Americans) is a service to Allah. Should we really shut out all the people who just want a better life, because a few of them might be a threat? Immigrants built America; shall we stop building America because we're afraid? We're all armed to the teeth these days - couldn't we handle a few jihadis, if they did show up?
As for their threat to American jobs - most immigrants take jobs that Americans wouldn't do, even for better pay. How much would it take to make you pick vegetables by hand all day, stooped over in a field? Or pack chicken carcasses on a factory line? Immigrants don't threaten American jobs; automation threatens American jobs. But that's another post.