I mean it's perfectly intuitive to me that a pyramid of rocks isn't going to collapse, so you don't need a container to hold it in. The closer analogue would be

I mean it's perfectly intuitive to me that a pyramid of rocks isn't going to collapse, so you don't need a container to hold it in. The closer analogue would be pouring sand in to a tube vs. pouring it in to a bottle that widens below the neck - it's intuitive to me that the bottle would be under enormous strain to hold the sand in. (And, incidentally, the part of the bottle where it widens would be applying extra "weight" to the sand below it)