Because ponifying the principle god of a religion predominately observed by the classical steppe nomads of central Eurasia and the Mongols just makes TOO much sense.
 
So, since not many people probably know who Tengri is, unless you’ve played Crusader Kings 2: Tengri is the principle God of the Tengri faith. Tengri lore stating he created Bai-Ulgan, who created the world. If I read right then he basically did this because the White Mother - Ak Ani - told him to.
 
Tengri is often depicted as a pure-white goose who flies over the seas of time. He is also hailed as Lord of the Blue Sky. Those who study religion place him in striking similarity to the older proto-Indo-European god Dyeus.
 
In the Turkic (Turkic in this case not inclusive to the Turks of Anatolia, but the general Turkic peoples from Turkey, to the Khazaks, or the Turkic peoples of Sakha) mythology from which he originates, he is responsible for the growth of plants, clap of thunder and lightning, or anything grandiose (such leading to anything being large, grand, or divine, as being referred to as “Tengri”).