Because you can't make a stable mountain if you haven't even attempted making a regular hill out of molehills. Making regular hills first will give you a sense for what kinds of structuring goes into a stable hill, and experimentation with various sizes of hill will give you a sense for what kind of engineering is required to make ever-larger mountains, and thus the sort of engineering that goes into mountains made out of molehills, before you finally get the boulders, trees, and snow you need to make mountains.