I think it is as you say... private tracts of rural land. Denser population areas and the difficulty to find tracts of land large and suitable enough to conduct stages that are not riddled with DZ'z, FZ'z and punctuated by nutralized zones (connection sectors on the special stages - commonly public roads). Argentina has a large beef cattle industry and agriculture to consider. The first Dakar in Sth America in 2009, I (IIRC?) they had one stage which travelled south of Buenes Aires. The dense bush and vegetation, combined with the fesh fesh (deep dust) made the majority of the stage for the later bikes, most of the autos and all of the trucks, like convoying at pace, down a dusty vegetated tunnel... unable to push forward through the pall of dust.