The main focus of the military isn't the capacity of learning, understanding, or reasoning.
Nor should it be. Soldiers don't necessarily need a capacity for learning, understanding, and reasoning. They don't need to know why...just how. Many of the higher ups know why...but your mid-level or grunts need to know how to do things....not understand why or what's going on.
Intelligent people are often taught to exercise free will, free thought, ways to solve problems no one has mentioned. While I'm not saying the military doesn't have these people. Most people don't do things patriotically. They do things to have family, make money, live a life. Some soldiers do things for patriotism, for country/service. But the majority of intelligent people find the best way to do things rather than the way you've been taught.
They work for big companies as execs, lawyers, scientists. The military has a small share of all those things...but they're not the best of each field at those things. Nor should they be.