You can say the soldier is nothing without all the rest of the infrastructure. Still, since the soldier is the one who takes the most risk of personal harm or death, its easier to imagine replacing any of the other jobs in the larger scheme of things. If everyone in your group decides its too risky to be a soldier, you are vulnerable to another group which thinks differently.
Guess that's the obvious part, how it is a rationality quote I'm not sure. Maybe its rational to put soldiers in a special and revered category given how important they are, for now at least.
I remember Star Trek TNG had an episode about a sort of progenitor humanoid race that had at some point in the past seeded parts of the galaxy with its DNA. So that was at least an attempt to explain why all the races were so similar. Even so I find it hard to get into any SF where alien races are obviously just subsets from human culture: the warrior race, the neutral race, the science race, the trader race, etc.