That is a good question, perhaps if we could one day determine their real purpose, we could perhaps learn the correct answer. I suppose that people in 4000 years time may ask "why did they build skyscrapers". We have our reasons today but perhaps by then they build downwards, away from possible damage from storms, who knows? There are so many, distributed throughout the world, that there must have been some allure to them. They were built by different cultures with different religions, at distances apart that defy reasoning of collusion, yet there they stand. Obviously some believe in alien visitations to explain them but many believe this too far fetched. Eventually discovering their original use, not that of burial, will make their popularity clear. Until that time we can only look and wonder on their spectacular beauty and marvel at the forgotten technologies that built them, puzzled at how such a great art could be so completely forgotten by a whole species.