Salt, like sand naturally forms a conical pile when you try to collect it in a heap. If you try to fight this tendancy and collect it into a cubical shape you have to reinforce the walls of your container in order to handle the outward pressure of the salt. Building reinforced walls is MUCH more expensive than simply allowing the salt to have its way and building a cheap conical building around it.
Such a simple answer, yet I can’t help wishing it was more, well, salacious. Not that my mind’s in the salt cellar, you understand, but neither am I a Shaker.