Because legally they have no recourse to tell if they are illegal, in many cases. You have what appears to be a legit SSN, you have a drivers license, you are willing to work the job. Maybe in some cases because you are okay with being paid in cash, with no paper work.
We have discussed somewhere else on the forum how easy is it for illegal immigrants to use someone else’s SSN and how there is no real government set up for the employer to check on it.
I work in a job that checks SSN’s, and the number of people who use fake ones is amazing. Some using more than one. Now, other than getting the job there is no way that is a benefit to the worker. They can’t file for a tax refund using that fake SSN.
I think also that illegal immigrants find out where they can and cannot work without being harassed, and they work those jobs for those employers.