The difficulties getting to the border, crossing and then being able to stay weeds out the dumb ones. Thus our immigration laws tend to be an intelligence test so that only the smarter ones stay successfully.
I don’t believe we are getting the brighter ones (the “cream of the crop”) anymore; it seems those can actually make a living back in their homelands (unless you take extreme cases like Venezuela or Cuba), and we get the dregs. Certainly in my area; we’re importing Hispanics here who are functionally illiterate in SPANISH. They aren’t needed as workers in these northeastern states with few jobs; they are needed as consumers of housing and the public school systems.