>The US doesn't just let people wander in. The US-Mexico border is incredibly militarized, with multiple lines of checkpoints on either side. [...] It's closer to the DMZ than it is to a typical international border.
Unless "multiple lines of checkpoints" actually means "multiple booths so visitors can be processed in parallel", this is most certainly false. Here's the San Ysidro boreder crossing in Tijuana:
You can see a wide fanout to a bunch of booths, a secondary screening area north of that, and some administrative buildings. That's not "multiple lines of checkpoints". It's just one checkpoint. There might be more drug sniffer dogs than the average border crossing elsewhere, but for the typical traveler it's otherwise not that much different.
San Ysidro is a port, only a small part of a much larger system run by the US CBP that extends up to 100 miles into the US. Here's the map of checkpoints for San Diego sector where San Ysidro is [0]. You can see an outdated map of the commonly used checkpoints for the whole US [1]. There's also a bunch of invisible infrastructure hidden between these checkpoints, from ALPR to facial recognition systems, not to mention regular patrols by USBP. Each sector's run a bit differently in practice, depending on various constraints and the different kinds of crossing they see. Tucson sector uses more mobile checkpoints because of congressional limitations for example, while Yuma sector focuses more on deep-desert patrolling in cooperation with tribal authorities because that's a well-developed coyote route.
A reasonably similar system of checkpoints exists on the Mexican side as well, oriented against people heading north.
Unless "multiple lines of checkpoints" actually means "multiple booths so visitors can be processed in parallel", this is most certainly false. Here's the San Ysidro boreder crossing in Tijuana:
https://www.google.com/maps/@32.5423767,-117.0289817,331m/da...
You can see a wide fanout to a bunch of booths, a secondary screening area north of that, and some administrative buildings. That's not "multiple lines of checkpoints". It's just one checkpoint. There might be more drug sniffer dogs than the average border crossing elsewhere, but for the typical traveler it's otherwise not that much different.