If you make it possible for a large enough population of immigrants to be legally permitted to work, you create a situation where employers can remain within th

If you make it possible for a large enough population of immigrants to be legally permitted to work, you create a situation where employers can remain within the law and only employ people who have legal right to work, meaning you can afford to actually enforce the law preventing employers from employing illegal workers (without as a side effect completely shutting down agriculture, construction and cleaning services across several states). THAT reduces the incentive to enter the country outside the legal framework, since all the jobs are going to people who have the paperwork. People do not want to go to live in places where they have no economic prospects.