Why are borders such a big deal?

I don't get it. I understand that we have borders as a way to have different sets of laws and separate different governments from each other. But why are people so obsessed with making the borders keep people out, or restrict what can come in, tax goods moving between them, etc.

Borders aren't real. At least not physically. If I cross state lines I don't see a literal giant black line dividing, say, North Dakota from South Dakota. In fact I wouldn't know there was a difference if not for the sign saying "Welcome to South Dakota"

For some reason humans can't conceive of a world where we all live on the same chunk of land and we somehow have to separate each other.

Shouldn't humans have a basic right to live where they have the best chance of survival? It seems like borders are just an excuse to exploit people along arbitrary lines by making up more rules to control humans who were born in the "wrong place".

I must be missing something. All of this effort around nations and patriotism over "I live here so I'm better than you" or "those humans suck because they live on THAT chunk of land instead of this one"... we're all the same species and yet we collectively seek out ways to divide ourselves and place people far away "beneath" us. All for some lines on a map.

Thanks for reading my Monday rant.