I’m not condoning Turkey’s actions, but this reasoning only works assuming there is a functioning government capable of securing its own territory on the other

I’m not condoning Turkey’s actions, but this reasoning only works assuming there is a functioning government capable of securing its own territory on the other side of the border and that is not currently the case. If hostile paramilitary forces were organizing attacks on the other side of the Mexican border at a time when the Mexican government and military itself had collapsed, I can promise you the US would have no problem establishing a safe zone beyond its formal borders.