Open borders violate property rights and require state owned land. There is no "right of the individual" to enter private property of other people. It takes state violence to maintain and rule over state owned land in order for the individual to enter another country. State owned land should revert back to the individuals and become their property right. Immigration then will be a matter of mutual agreement between owners of land and other individuals and would not require collective i.e. statist ownership of land.

In a libertarian world every private property will be a border and every border would be as closed as your front door.

Statists eagerly promote open border policies because they see it as a lever against property rights and for redistribution (tax theft). Third world refugees have nothing and they're willing to vote for them the property of original citizens (white Christian males are the most hated by the system because the have the most) and also they're not so much civilized thus they're easily manipulated (see who rules over them in the Middle East). So statists want to substitute whites for non whites. Plus making wars in the Middle East reinvigorates the health of the state and sends back home terrorists and state land dwellers (refugees), both end up increasing state power. Statists want third worlders in the west, they want tyranny and exploitation. No open-border supporter will ever care about real individual rights which is property rights.