Some mountains are entirely natural, formed solely through plate tectonics, and indeed many mountains can be found in lands where no Aggron dwell. And while many have noted that Aggron are strictly territorial and each one calls a single mountain home, few understand the reason for this behavior: Aggron are mountain-builders.
Even for a pokemon as large and resilient as Aggron, building mountains is a nigh-eternal task. Even if it did not take decades for their hills to reach the level of climatic variation necessary to be termed mountains, no Aggron ever considers their mountain complete. They continue to build higher until they die, seeing in their mountain's growth a lifetime of accomplishment, one which they must defend from raiders – hungry Tyranitar, other Aggron looking to seize the dirt and stones used for their own mountain, or humans wishing to do the same to win building materials for their cities. They cultivate them as another form of defense: with poor planning erosion can deal damage faster than a pokemon can build, so trees are used to make an Aggron's mountain ever taller. When an Aggron dies, they will try to pass the mountain onto their children, but a mountain's size is an expression of the ruler's power, and few can match the greatness of their parents when still young.
In every ocean are found islands consisting of nothing more than a mountain rising out of the water. This is what happens when an insular Aggron is able to claim total victory; only the vast size of our homelands gave Man and other pokemon room to develop in a multiplicity of terrain. Had one ever triumphed, we would live on a single mountain ruled by an Aggron king, his throne at the summit in the heavens.