Stripped of all superfluous hyperbole about human rights, sovereignty, and America's image on the world stage, there is one and only one reason that the wealthiest and most powerful nation in the history of history can't manage her borders.
They don't want to.
"Undocumented migrants" have the lowest rank in that "reserve army of the poor" old Karl Marx was scribbling about 150 years ago. As such, they provide a valuable service to the elite capitalist class who have been managing the affairs of The Republic since the time of the founding fathers.
They are absolutely essential in ensuring that real-life working Americans never get too uppity. After all, the lads at Paramount Landscaping in Bergen County won't be getting too boisterous in their demands for a living wage as long as their boss can assure them that there's plenty of hard-working Guatemalans around just waiting for a chance to do their $12/hr jobs for $6/hr.
Landscapers and roofing contractors and farmers and others who employ this lowest rank of the working class are hardly the "capitalist elite" of course. But trickle-up immiseration works way better than trickle-down prosperity. Downward mobility has trumped upward mobility in America since long before Donald Trump.
Just look around you.
The fact that an ersatz billionaire reality TV star promised to build a wall at the Mexican border speaks only to the calibre of his image-management team... as in he really cares about American workers and that wall is gonna make America great again!
Of course it is!
You don't imagine for two seconds that Donny J is interested in ensuring that all the grounds-keepers at his various properties actually take home a living wage, do you?
And if you thought Hillary cared more, you're an even bigger idiot.
Happy 4th of July, neighbours!