You don't "maintain stable social relationships" with the whole horde, you maintain them with your small unit. And the military hierarchy exists so that people have to coordinate only a limited number of entities: if you're a grunt, you need to coordinate only with people immediately around you; if you're a mid-level officer you coordinate a limited number of platoons, if you're a general you coordinate a limited number of regiments.

A soldier does not meaningfully "co-work" with the whole army.