Do you think that the reason they aren't going after employers is simply influence in Congress? If employers pay too high a price to hire undocumented migrants, they won't do it. No jobs for them, and they won't come here (or stay here). Seems really simple, but no one in Congress is even suggesting such actions (that I'm aware of). Why?

Because they didn't like what happened when they went after employers earlier in the term.

Remember the Hyundai raid in Georgia, back in September? Caused all kinds of problems for them when their efforts ended up jeopardizing the operations of the factory. They did the same thing with farm raids, and restaurant raids, and hotel raids. ICE went big on those during the first few months of the Administration, with predictable results:

The Trump administration has abruptly shifted the focus of its mass deportation campaign, telling Immigration and Customs Enforcement officials to largely pause raids and arrests in the agricultural industry, hotels and restaurants, according to an internal email and three U.S. officials with knowledge of the guidance.

The decision suggested that the scale of President Trump’s mass deportation campaign — an issue that is at the heart of his presidency — is hurting industries and constituencies that he does not want to lose.

https://archive.ph/NgpAk#selection-687.0-691.208

Trump is a very transactional, and in some ways very practical, President. He wants really strict immigration enforcement, but if it starts to have outcomes that hurt him or hurt the constituencies he cares about not losing, he will jettison ideological purity in a New York minute. So it's not so much that Big Ag or Big Hotel or Big Restaurant has influence in Congress (which of course they also do), but because Trump doesn't want to be seen specifically targeting businesses in a way that he will get blowback for.