TOTW: why are we so obsessed with borders?

In light of what is happening in America and Europe; the dire treatments we are seeing of people trying to migrate from South America into North America and migrant ships anchored off European shores being refused permission to dock. We wonder why we are so obsessed with our borders and why would we rather put people in cages or leave them floating on the Mediterranean Sea then facilitate them escaping a dire situation in pursuit of a better life? Most people just want the opportunity to contribute to society and enjoy a decent family life like the rest of us.