I doubt you have many Mexican locals living next the the border literally walk across the border and then travel to the interior of the country. Most of them are from far away places and endure quite a challenging journey to get here. I live in San Jose in a neighborhood with many older Vietnamese boat people. Columbians have been moving in. There isn't an easy way to get from Columbia to Mexico by walking.
Crossing the Darién Gap: Migrants Risk Death on the Journey to the U.S.
Yes, the trek is hard for many. But physically hard doesn't imply cognitively hard or selective for cognitive traits. These paths are well trodden and the difficulty is purely physical.
Crossing the Darién Gap: Migrants Risk Death on the Journey to the U.S.
https://www.cfr.org/article/crossing-darien-gap-migrants-ris...