Why would we spend time and money on enforcing immigration employment laws to catch people who are not legal to be employed? Why not stop them at the border and put pressure on Mexico to get its crap together and stop blaming us for being a **** hole country that people are fleeing?
It's not an either/or. You secure the border, pressure Mexico, and go after employers.
Why go after employers? Because it's easier to make an impact and because they're bigger villains - same reason going after a drug dealer makes a bigger impact than going after a lone dude smoking a joint at a Willie Nelson concert.