An employer who hires an illegal and pays him or her the same amount they do a legal employee saves 15% Social Security tax, workman's comp, insurance, and doesn't have to worry about paying unemployment benefits for the employee if they file for unemployment two years later. They also don't have to worry about being sued for phantom back injuries or perceived slights or unfair treatment.
I just started a sole proprietorship business (no employees), and within two weeks of starting up, I got a letter from the state telling me if I hired anyone, I'd have to pay unemployment compensation and workman's comp. As best as I can figure, if I were to hire someone and pay them $30,000, they'd have to be able to generate around $50,000 in revenue for me to break even.
That's not just a problem with illegals. Medicare and Medicaid patients reimbursements are grossly inadequate and the cost of treating them is transferred to other payers also.