Why do we worry about national border in the first place, and why do they need to be guarded in the first place?

Why do you lock the door to your home and not leave it open?
Why do you call the police when someone breaks into your home or tries to rob you personally? And if you don't or wouldn't do that, can you expect the same attitude from those around you that you say is yours?

Let me ask again: Would you fully accept that you are thrown back on your immediate surroundings and thus on your own skin? Without relying on an existing border with its law and order, you'd have no other chance, is that correct?
Where would you draw the line, one mile, ten miles, fifty miles away from you? Or to the street you live? Or around the place you work? Where?

An order exists already but your former government did not execute it properly. That created such agitation to the other half of the population (to use a simple number) that it now is on drastic mode. But why is it on drastic mode? Does it come out of nowhere? Is there a reasonable explanation, you can give? Since you seem not to have accepted mine in pointing towards "time" and "numbers".