There is no excuse.
Borders serve no useful purpose in a globalized economy. Can you imagine how terrible it would be if the states had hard borders between them?
People act like having open borders mean invading marauders of foreigners, but did everyone from Ohio invade Maryland because of the open borders the states have? In reality, most people would stay where their family and relatives are.
"My dream is a hemispheric common market, with open trade and open borders, sometime in the future with energy that's as green and sustainable as we can get it, powering growth and opportunity for every person in the hemisphere." - Hillary Clinton
Let's start by making annexing the states of Canada and Mexico into the US, and work other countries from there.
Do it.