Many people want a simplistic philosophy that they think explains everything and can be applied to everything.
"Libertarianism" is one of them.
I meet people who think there should be no borders at all because they think they are "imaginary."
Guess what? Property lines are imaginary, too.
And the only reason property lines can be enforced is because of violence, or the threat of it.
Fortunately, most people will respect property. If they didn't we wouldn't have a viable civilization. But there are that small minority of people who don't respect property, including your body. Ultimately the only thing that works on them is the threat of violence, or violence itself.
Of course the greatest offender is the State. But the fact remains there has never been a society that didn't have government, and anarchism is not the solution, because there were always be a sociopathic gang that decides to rampage through society. And these problems are worse in a "multicultural" society, because different tribes cannot share the same land - just the way "Hispanics" want blacks out.
I lived in Albuquerque for four years. "Hispanics" hate blacks, just the way blacks hate Asians and abuse and attack them.
In fact there may be no solutions, just trade-offs.
The problem is these open borders types don't understand human nature, which in my book makes them leftists who are trying to overthrow ancient wisdom and impose an utterly unrealistic belief system on people.
I'd hate to end up in the kind of society described by Neil Stephenson in The Diamond Age - heavily armed tribes eternally defending themselves against predators.