You can still visit Butch Cassidy & the Sundance Kid's ranch cabin today just off Ruta Provincial 71 in Cholila, Argentina. Here's the location on Google Maps:
https://goo.gl/maps/pJuzdodbpJeejLEu7
It's totally bonkers that they met Hiram Bingham while he was on his way to go discover Machu Pichu. South America is apparently a much smaller place than it would seem!
I think it’s more to do with there being established routes and nexuses, much fewer people than today and they probably didn’t feel the need to go off-piste.