The question is why American employers hire illegal immigrants.

We can assume that they have some economic necessity or at least benefit.

To punish employers, we would need to establish the police force continuously monitoring all employers. It requires money. Then, assuming that employers have profit from hiring illegal immigrants, we can deduct that they would try to bribe the policemen watching them. To prevent that, we would need someone watching the police and, next, someone watching those who watch the police.

In short, to enforce our current immigration law, we need a Soviet-style police state. It would end up as the Soviet Union did.

You need to come with better ideas.