I think building a grain store in the shape of a giant pyramid wouldn't work anyway. The grains at the bottom would be pretty much unusable and likely you couldn't get them out except from the top because of the weight. I do wonder if someone could work out the math though :-)
If you make the bottom of the pile accessible in some manner (like with a door), it's more efficient to withdraw the grain from that point due to gravity.
The Great Pyramids are around the 50° mark, where modern wheat's angle of repose is roughly half that (27°). That's not to say a pyramid-like structure couldn't be used to store wheat—just that the shape of these particular pyramids don't directly mimic the free-standing angle that a wheat mound would form.
I doubt they were grain stores, but elevators in the USA store grain and corn like that all the time in a pyramid shape and it doesn't hurt the grain / corn (other than moisture).
There is a "ski Indiana" photoshop showing a skier on one of these grain hills.
