Because it can't be a forethought. D&D is a game about individual characters doing things in the world, exploring, roleplaying, etc. A game designed to do that well can't also do mass combat well. To do mass combat well, you need to buy a game that is designed to do only mass combat.
I've had DMs tell us at the beginning of a campaign that if mass combat happens, we are going to switch to X or Y mass combat game to run them. That to me is the best way to run mass combats in D&D. You play D&D until a mass combat happens, and then you pull out Warhammer Fantasy Battle or something.
There is, but they are not a company that is willing or able to make a good mass combat game that is separate from D&D, so they keep failing.