This was especially obvious during the last administration* when ICE was raiding businesses left and right to deport people and as far as I know, almost never went after the meat packers and farms and other businesses that knowingly hired the migrant workers. As long as the employers don’t see any penalties or they’re so small as to be the cost of doing business, there will always have a large pool of undocumented immigrants who will replace the ones deported.
I think if they were actually investigated the well would run deep with plenty of employers actively helping their new hires commit fraud to get past their I9 verification. It’s unfortunate that this approach has never been politically viable because I suspect a majority of the population is willing to approach illegal immigration humanely while punishing the actual lawbreakers upstream to address the core economic impacts.
* It was obvious to anyone paying attention during the Bush, Obama, and Biden administrations too but the media focus during Trump’s made it especially stark how little enforcement was going on at the employer level.