I apologize as it appears I stand corrected. But still, prosecution is next to impossible as you have to actually prove the case.
Prosecution is rare because the penalties are so small and the prosecutors make their careers convictions that produce headlines, not from putting criminals away for 6 months. Were they doing their jobs it might be hard to prove the criminal entered the country illegally but it would be easy to prove they “Eluded examination or inspection by immigration officers” while here and the criminals would be looking at doing 6 months for their crimes.
If the criminal is also working he has committed perjury when he falsified and signed the I9 form all people must sign to be employed. The perjury charge is a felony and a no-brainer to prove. Perjury can get a perp 5 years in a federal pen. The federal prosecutors are just not bringing the charges.