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Silica dust will kick up off the nearby dunes, carry in the wind, but due to its weight it’ll be less likely to erode higher elevations. So the tip top of the pyramid is high enough up sand in the wind won’t reach it as easily so it erodes way slower.

That, and the pyramids were stripped for building materials later on. Lower levels were easier to access, so people didn't generally bother going all the way to the top, except for the ones with gold covered peaks.