Blame the fact that it's so difficult to immigrate legally yet an overwhelming number of businesses rely on cheap immigrant labor. This is a feature, not a bug, because they can pay the illegal immigrants less and abuse them without fear of reprisal because if the employer gets any attitude about it, they call ICE and have the worker deported and replaced with the next struggling desperate immigrant.
There is no illegal worker in the US that would rather be an illegal worker than a legal carded one. The system is stacked against them because it saves businesses money to do so.