One factor is that it's simply become too expensive (or jobs don't pay enough) for most couples to have several kids, if one is not on the gov't teat. My wife and I planned to have 3 children (2 by us and adopt 1). But the "economic reality" of 2008 - 10, for us, a very middle class family (the industry I was in collapsed), meant we could only have 1, and now my wife is past her childbearing years. (Granted, we got married / started late.)
But, that's an aside. Presently the US has a tremendous labor surplus.
Key point. The Mexicans are all on that.
Cuz ya know....the gringos got all the cash, so why not.
The concept that it might all run out some day is apparently beyond them.