From: LobsterDandy | #141 The game essentially stopped for fighters at level 9. After that they would gain 2 hit points and improve their THAC0 and that was it. Pretty much.
Here's the thing about running an army. You can't do jack **** with it. I mean, you can't take it into a dungeon, you can't really use it unless you're fighting a war, which, let's be honest, doesn't ****ing happen in Dungeons and Dragons. You might fight in a war, but you're not really sitting back in your castle, sending orders via runners because that's not fun. I guess you could use it as some kind of political clout, but I don't know.
It goes back to Gygax's complete misunderstanding of what people were actually going to do with his game.