Depending upon whom you believe, there are some 10 to 12 million illegal immigrants in this country. Why? (I’ll warn you ahead of time: you won’t like the real answer.)
This traces directly to inaction by Congress since the end of WW2, when they ended the Bracero program with the mass deportation of Mexican agricultural workers.
Long form follows:
There are H1A visas for technical people, and there are H2A and H2B visas for unskilled labor, intended to be work permits for mostly-Mexican laborers. These H2A and H2B visas cover the laborers, plus the dependents they bring with them. H2A visas are specifically for migrant farm workers, and H2B visas are for all the other migrant worker trades, such as the truly grungy stuff at construction sites, concrete work, road work, lawn care guys, and toilet cleaners at motels, etc.
Because these jobs are both low-paid, and very hard and unpleasant work, almost no Americans really try to apply for such jobs, despite what some claim. We are awash in fake news about this issue, among many others. You can recognize a fake news echo chamber by the lack of divergent opinions, it really is that simple.
The low pay for the immigrant workers is a vicious cycle: because most of the workers are illegally here, their employers simply extort labor at very low pay from them. This is immoral and unethical, but VERY widespread. If these workers were legal, pay for that work would have to rise, and more Americans might even apply for such jobs.
These workers are a huge factor in our economy: reportedly around 15% of construction jobs, and apparently almost all the crop harvesters we have ever had since WW2. If you deport them all, important sectors of our economy not only crash, but you will go hungry because of high-priced foreign food imports. Now that's the real facts, unpleasant though they are.
Why are these workers mostly illegal and thus subject to extortion into wage slavery? Because the worker permit visa quotas controlled by Congress are completely out-of-line with the "ground truth" of our economy. The demand and corresponding need is there, the accommodation is not.
According to the Brookings Institute, the annual cap on H2A and H2B visas totals to about 125-150,000. That's roughly a factor-of-100 out-of-balance with reality: the 10 to 12 million that are here doing the work, and paying taxes on their meager wages, despite what some say.
What no one wants to hear (but the painful truth will set you free, when political lies won’t): we brought this on ourselves; more specifically, our Congress did, with over 7 decades inaction on this issue. That is utterly inexcusable.
Worse, some of them run for re-election promising to do the wrong thing about this problem! But we keep electing and re-electing all the idiots that did this!
So, stop re-electing them! Elect instead somebody who will really fix this, by actually doing something about the out-of-balance visa quota system. You’ll see this problem melt away in a very few years, if this imbalance is corrected.
(And by the way, fixing this permanently fixes the DACA problem, as well.)