The Mall is the product of John, Rose, Dave, and Jade creating a universe
All the Kids really wanted was to hang out, preferably in a big Mall to show Jade the world and they got it. That's why the Mall has 25% of it. The Mall is actually a battlefield for the Immortals.
Due to the nature of the Mall being combat-based, it can only be a safe assumption that it was made specifically for the Gathering, since the most inventive and skilled Immortals have an advantage with the abundance of materials for improvised weapons.