A good deal of the reason is because a significant portion of the American workforce is locked out of many of the jobs that illegals fill because of government regulations and liability insurance exclusions on underage workers. Specifically, those under 18. For instance, when I was in high school, one of my classmates worked a full-time shift drilling oilwells. Absolutely not an option for a 17 year old today. A case in point--My neighbors own an art gallery & needed someone to run a weed-eater & hedgetrimmers around the landscaping at the gallery. They talked to us about hiring our 16 YO son but later discovered they could not legally hire anyone under 18 to do that type of work. Many of what were traditionally teenage type jobs, like mowing lawns, are now off-limits to those workers. Illegal workers fill the void.
Just two days ago, I noticed a jobs posting on the door at my local grocery. Needed are workers in several departments w/ the addendum that those over 18 YOA are especially encouraged to apply. For sure, I would imagine that the store wants more mature workers for lots of good reasons, but other reasons are because of state laws that strictly & rather severely limit the number of hours underage workers can be on the job and the additional & burdensome amount of paperwork the employer has to complete documenting that underage workers are not working past the legal limits of hours on the job. The end result is that even for jobs that under 18 workers could legally perform, they don't get the job because the employer just doesn't want to have to piss around w/ all the extra paper & regulations involved in hiring them.
Up until the beginning of the illegal worker invasion, most of the seasonal agricultural work was done by young men ages 15 - 18 during summer break.
(In fact, that is why we have a summer break to begin with, so that children could stay at home and help bring in the crop harvest.)
Now, the only thing we are harvesting in this country (besides the lettuce, corn, chicken, peanuts, etc.) during the summer is gang murder and millions upon millions of illegal workers.
Speaking of the legality of not hiring a native American youth under 18 ~versus~ hiring an illegal immigrant - really makes one wonder who is BULLCHITTING who is this country!