我的殖民者为什么不去看看那里发生了什么？

I would like my human colonists to settle on the high plateaus (2500-3000m above sea level) of my planet thus never meeting the civilization living in the lowlands (in a huge forest so they aren't visible from above), until settlers find evidence of them and decide to see what's going on down there. The thing is that I need a reason why they wouldn't go to those lowlands earlier, so something that makes the descent difficult but not impossible, and also why my civilization wouldn't develop on the plateau. Here are the main ideas I had: I would like my human colonists to settle on th