It is all about egos. Dubai wanted to be top city of everything including world's tallest building, world's largest shopping mall, world's tallest hotel, world's largest indoor ski resort, world's largest man made islands, etc. Dubai has another plan to build another world's tallest building called Al Burj which is over 1200m tall and more than 200 stories.
Dubai is doing exactly what New York City did in the 30s. Chicago is building world's third tallest skyscrapers, Chicago Spires which is over 2,000 feet tall and 150 stories. Las Vegas is trying to build Crown Las Vegas which is over 1,800 feet and hope to be world's next tallest hotel but having an issue with FAA right now. FAA haven't approved this proposed tower yet.
Billionaires can afford to construct world's tallest buildings.