Jobs are plentiful in the US just the US Citizens don't like those.
So they employ illegals.
The aim of a citizen in a country like US is to keep going forward, You don't think anyone who had better opportunities before to now go into jobs they won't have ordinarily done.
As a citizens of US many of them already have great options but as a immigrant which most are coming from countries with little or no opportunities they actually have no choice and anything is quite good for them.