The correct answer to this open question is the following.
The Chavin and Inca civilizations were confined to mountainous areas because that was the best way to be isolated in order to avoid ant invasion from outer tribes.
For instance, the impressive city of Machu Picchu was built at the top of the Andean Mountains in Perú. Its impressive architecture, the heavy pieces of rocks used and the way they prepared the land to grow crops in such difficult conditions, could be even harder if the Incas had to fight foreign invaders. So yes, being isolated in this way represented a major advantage for them