Because it is an ancient Sumerian title. The four corners of the world refer to geographic locations, and basically mean as much as ruler of the world.
King of the Four Corners of the World, alternatively translated as King of the Heaven's Four Corners or King of the Four Corners of the Universe and often shortened to simply King of the Four Corners,was a title of great prestige claimed by powerful monarchs in ancient Mesopotamia.
Though the term "four corners of the world" does refer to specific geographical places within and near Mesopotamia itself, these places were (at the time the title was first used) thought to represent locations near the actual edges of the world and as such, the title should be interpreted as something equivalent to "King of all the known world".
The title was first used by Naram-Sin of the Akkadian Empire in the 23rd century BC and was later used by the rulers of the Neo-Sumerian Empire, after which it fell into disuse.