Gosh, I wonder why. (sarcastic smirk :o)
Just how many men & women would it take to close both borders?
What's with the "both" thing? You'd also have to close the beaches, too. (The Coast Guard can't handle the existing maritime immigrant smuggling as it is; closing the land frontiers would move that traffic to the sea.)
The United States has over 19,000 miles of frontier, much of which is thinly populated. Securing that much perimeter would take a significant chunk of the US citizenry.
Gosh, I wonder why. (sarcastic smirk :o)
Well, Einstein, that would be because hardly any illegal aliens are flooding across the Canadian border. (smirk)