How can anyone.....
Think back to '89.....the end of "history"! For decades highly guarded borders fell and people were able to move again....open borders were seen widely as a good thing, meaning freedom!
The other big part was that the economies internationalized....wandering worker masses meant less production costs because of lower salaries....for a few decades lotsa people profited from open borders and open economies, from unified countries....nobody thought that would ever change, but change it did!
The age of globalization is now ending, the economies are about to be re-nationalized....hence the anti-immigration movements are winning....at the end will be re-build borders.
But you can't say that only leftist morons were for open borders....quite the contrary....they did build an unholy alliance with the great capital, the big firms, their sworn enemy, to keep the borders open and the cheapest workforce coming. I doubt they have accepted and acknowledged that fact till now actually....