It's in the damn name. Immigration enforcement. We have a legal immigration process in this country, many try to come here illegally. Without a border, you have no country.
Permeable borders abound in nature. Trying to obstruct the natural behavior of migration is as foolish as trying to halt the tides and arguably causes worse economic distortions than it solves.
The "natural flow of migration"?
Most of your "illegal immigrants" are legitimate visitors who overstay their visa.
1. https://news.ycombinator.com/item?id=19547565
2. https://www.theatlantic.com/politics/archive/2018/11/trumps-...
3. https://www.nytimes.com/2018/08/09/nyregion/melania-trumps-p...