Why Tibet is known as the roof of the world?

The land of Tibet is the highest plateau in the world. Plateau is flat land, much higher than sea-level, like mountain which have flat land at the top, instead of peak.  It occupies an area of around 1,000 by 2,500 kilometers, and has an average elevation of over 4,500 meters.  It is the highest and biggest plateau, with an area of 2.5 million square kilometers. Because of this large flat area over the sea level Tibet is known as the roof of the world.