Ummm, America? I do not need any paperwork to walk wherever I want to, most of Europe is that way too. I’ve crossed borders on foot a couple times in Europe. It’s not a big deal. In my old life I flew over a lot of borders - you cannot see the line usually from the air. Stressing about imaginary lines is unbelievably silly.
Look, humans are by our very nature nomadic, we traditionally used to have a “seasonal round” where we lived in multiple places in a given year to have more access to better food/water sources etc. Being able to travel where you like is a fundamental part of being human, borders restricting that are what’s weird, not the other way around. Restrictions on travel are a relatively new phenomenon. The difference is it was just physically harder to get places 1000 years ago, and that formed its own sort of barrier.
Someone else said that you used to not need ID to even fly before TWA800, I don’t know if that’s true, but I do know that you could walk up to the gate when I was a kid. And why should you need an ID? Tracking every movement of every person doesn’t grant any sort of security to anyone.