Because they are trying to highlight how the Dems are doing nothing to protect them by failing to fix the border.  They will holler and scream about it and Trump will sit back and say he has been ready and waiting to make a deal that would deal with illegals already here.    Plus, its such a small number of the million that fit the criteria that they will see the threat as too small to afford running and hiding. It be like saying we are going to arrest 5,000 people between the ages of 24 and 25 nationwide.  What is it, like 3-5% chance that it be you.  I don't think the goal is to remove them as much as it is to drive home the political point that Dems are refusing to make a deal.  However, it may also be that he sees some electoral areas that need democratic votes thinned out, but this is a questionable theory since lots of Latinos who have been here a long time support Trump since he is the one willing to make a deal and Dems are not.  Plus who knows better than recent immigrants how many bad guys are coming across the southern border.