It's simple: they're taking jobs from Americans because they work for 40 percent to 60 percent less.
When I was in college and working summer construction in the 70's, the hourly rate for unskilled laborers was $6 to $8. Using any conventional inflation measure (e.g., CPI, Average Wage Rate), today's rate would be $24 to $28 per hour.
Anecdotally, illegals are making $12 to $15 per hour today for uskilled labor, which obviously is 40 to 60 percent less than the rate paid to the largely American labor force of the late 70's.
... And the only reason thay accept 40-60% less is because they are ILLEGAL. Once they are given legal status, they'll demand an "equitable wage."
Take away the job magnet and those who are here to work will go home. Those who remain will be here either for the Welfare or the Criminal opportunities - these should be deported.