Re: @Tom Dial ... ...because we are a nation of laws
The checks and balances are there, but for going on a century or more the Congress has declined increasingly to enforce them, instead passing laws that deliver a lot of what most would consider legislation to executive branch departments and agencies. They pretend to oversee them but really cannot do so effectively.
There is no question that "immigrants" have, collectively, been to the good, whether or not "legal" at their time of immigration. The question of legality is, to a large degree, a result of legislative choices made at particular times and generally carried forward for years or decades beyond any justification cited for their enactment.
Moreover, immigrants' descendants have been more important than the far less numerous immigrants, something the advocates for "dreamers" and permissive immigration for the educated and already-accomplished (or the rich who can buy green cards for a half million dollars or so) usually overlook. One key attribute of immigrants, whether or not "legal" is their initiative and willingness to accept the substantial risk of failure that goes with moving to a new place, often enough with a different primary language and different customs. Not all of them succeeded, but quite a few did, for a reasonable definition of success. Their children and later descendants have done so even more, and there is no reason to think that won't be true of the current group of immigrants - again, whether or not "legal." The primary "victims" of illegal immigration, in fact, are the applicants who played by the rules and waited in the queue for their chance.