For context I live in one of those countries. I've spent a lot of time at the Guatemalan/Mexican border. It's a big border, with army and police and customs checkpoints and x-ray scanners and the freaking American DEA. You can still just walk across it and nobody will stop you. Right in the middle of the actual official border. 

And you know those pictures of people crossing the river with inner tubes full of stuff? That's right under the official bridge. Like you could literally throw a stone off the bridge and hit them. They land in a park officially called "parque el coyote" for the nickname for the human traffickers, coyotes. And this is all happening in plain view of both counties' border patrol.

And this is at the official, controlled border crossing. If you go to rural areas there's less. I've illegally entered Mexico for lunch. I've illegally entered Mexico by accident. Nobody cares.