I'm going to assume you mean the Americans who have been here awhile. I'll address your post that way. More Americans are getting college level degrees or going to tech schools and not needing to do "entry" level jobs and labor intensive jobs UNLESS they pay well. That's not too hard to figure. Which job are you going to take if you have a choice? The illegals don't have that option.
But like I said as an example - if bosses are paying illegals $2 an hour less than legals that adds up to $4000 a year. You may not do a job for 16000 but will for 20000. Pay the higher wage and you attract the legals, dont and you get the illegals. And obviously since not much is being done about the illegal aliens people will continue to hire them.
Quite a few years back relatives from Down Under said the same thing, immigrants are needed for the hard labor jobs because the legals wont do it. The legals can get better paying jobs elsewhere. That's what it's all about the money.