Around 12 million people are in the US illegally each year.
That's a dishonest phrasing which implies that there are 12 million arriving each year. Your later phrasing is more honest:
12 million people being present illegally
How is that acceptable?
How is it fucking acceptable to kidnap random people from the streets based purely on the colour of their skin, lock them up without access to a lawyer and deport them to a country they've never been to in their lives against judical instructions that nothing should be done until a court has been satisfied that the person actually is there illegally. Either everyone has due process, or no one has due process.
ICE aren't going after the bad guys. That involves work, investigations, actually putting officers into the dodgy bits of town. They're picking people up from their citizenship ceremonies - after someone has done things properly, has been through all of the hoops (green card etc.), their citizenship application has been approved and all that is left is the oath. ICE use these as easy pickings. They have been hanging around outside the courtrooms and immigration offices where people trying to do the right thing and apply for visas, green cards and citizenship go.
because for any normal and sensible person, it's unthinkable to commit such a crime in a foreign nation.
An "unthinkable" crime would be something like inciting an insurrection, launching a missile at an unarmed person who has been shipwrecked, raping a child, or falsely imprisoning thousands of people. No normal and sensible person would do such things, but apparently 77 million Americans are quite happy to vote for that. Overstaying a visa is utterly trivial by comparison.
So what on earth makes you think that 12 million people being present illegally in the US is not an issue?
I honestly couldn't give a shit. It's less than 4% of the population. With everything wrong with the world, do you seriously have this as your top concern? If someone commits an actual crime (being present illegally in a country is not a crime) then deal with them properly. 95% of the people picked up by ICE have no criminal past. Many of them aren't even illegal immigrants. Some are actual citizens - ICE will lift them anyway. Most are honest, hardworking, upstanding members of the community. More upstanding than the paedophile rapist-in-chief and the Proud Boys in ICE.
If you want fewer undocumented migrants arriving, how about you try not destabilising every country in Latin America? Venezuela is just the latest in a long list.