The terrain is really, really rugged. Steep mountain ridges isolate clans in their valleys, thick, hyper-tropical jungle making travel very difficult. Remember those documentaries, 'Lost Land of the...' ? In one, the expedition was trying to reach the temperate montaine forest above similar jungle. IIRC, it was taking them up to an hour to clamber each hundred metres of slope.
I've read that the Japanese lost an army into the mountains. The well equipped force headed South into the ranges, lost radio contact. They were not attacked or ambushed, the jungle just swallowed them.
To show how difficult the terrain, IIRC, an early orbital radar pass found an entire mountain range and valley complex which had gone un-noticed, un-mapped...