Because that's what actually happened. The main force of the Aztec army was from the core of the empire. This whole time you've been demonstrating a very poor understanding for how the Aztec empire actually worked or even what it was. The Mexica, Acolhua, and Tepaneca supplied more warriors than anyone else. I don't know why you seem to think all "Aztecs" were effete noblemen sitting in offices either. Most of them were commoners who were trained to fight from childhood and even the nobles were expected to fight and given military training as well. Priesthood does not exempt one from service either, these aren't Christian monks.