thats simple to answer, a pyramid is the mose stable shape for a large building, much more so than a totally vertical building, biggest at the bottom smallest at the top is a simple engineering principle, therefore it was common before there was modern technology such as steel frames for large buildings, thats why you dont see any ancient buildings with a 5x5 metres base and a 500x500 metre top, if they built one like THAT and it was still around then id be impressed.