Although the estimates vary in regards to how many illegal and unauthorized immigrants are residing in the United States, at the present time, it is fair to state, that there are at least 11 million undocumented workers in this country, and that number doesn't include those that were formerly undocumented or unauthorized that were able in one way or another to receive authorized documentation or amnesty to reside in America, and to even to get their citizenship.
The very first thing to remember is that America, has been dependent upon cheap labor almost since the inception of this country, of which, back in its founding days, slavery was legal, and therefore the cost of cheap labor for those procuring such, was fundamentally about buying peoples that had been forcefully extracted from other countries, to slave for plantation owners and similar, without those laboring having recourse to wages, or basically as Supreme Court Chief Justice Taney stated, that blacks had "no rights which the white man was bound to respect."
In today's society, there isn't any slavery, for not only are there no more slave auctions, but slavery has been Constitutionally banned from this nation. However, as in most countries, laws are not equally and equably applied, and further to the point, those that are not citizens within a nation, are absolutely susceptible to exploitation of the highest order, especially when those so hiring the undocumented workers, are well aware that those being hired, do not have equal or effective access to justice or the laws, and purposely make it their policy to take advantage of all those that have little or no ability to fight back.
This indicates something that most Americans seem rather clueless about, which is virtually all of the illegal and undocumented workers that are here in America, are actually here under the aegis, more or less, of industries that are dependent upon having ready access to cheap labor that can be exploited in order to make profits within their businesses, such as back breaking agricultural labor of all kinds, industrialized meat and poultry plants, hospitality services, landscaping, construction, and so on.
This signifies, especially in smaller and more insular communities, that the justice and policing arms of those communities are clearly aware of the presence of illegal workers within their communities, but will not interfere with the very labor that is necessary for the community and the businesses to prosper and benefit from; because without that labor, those businesses would effectively cease to exist and would no longer be profitable at a sustainable level, because their business model is specifically built around continuous cheap exploitable labor.
This means that the real reason that millions upon millions of undocumented workers are within America, is because it is very, very good for the business interests of America. Further to the point, those businesses aren't interested in changing anything within the current construct that they are working within, because it is the money maker for them. This so means, that while slavery is gone, its replacement, is built upon the purposeful and deliberation exploitation of undocumented laborers, and more than that, thrives in a nation that has the audacity to label itself as the beacon of liberty, justice, and freedom for all.