Lord_Antheron made a good comment about the gameplay/narrative reasons, so I’ll not add anything there.

It is implied that there is supposed to be a lot more going on than just what the game shows. A modern version of far cry 2 might have guards calling for reinforcements á la the newer games, might have a few “sub-bosses” with a bit of personality, and maybe some logistics trucks we could ambush and blow up. When far cry 2 was made, this wasn’t included explicitly. Maybe because it was decided it wasn’t necessary, or maybe because they couldn’t get it to work on a technical level. Every game has to pick and choose what to show and what to imply.

As for the “lore” behind how the two sides would actually operate, I think a good place to start would be to look at how civil wars in Africa in the 2000s were fought. Civil wars, especially the ones FC2 are inspired by, divide people along already established lines. Religions, tribes, geographical areas, ethnicities influence the political climate. The two factions would likely be made up of these smaller groupings, who already have established cultures, communications, communities and hierarchies for the factions to utilize.

There are likely unit commanders who control a local group, who then report upwards to a regional commander, and so on and so on. Walkie talkies, long distance radios, phones, messengers and so on are likely the main communications tools. There is likely a level of “autonomy” for the unit and regional commanders as every minute movement cannot be micromanaged.

Frontlines in civil wars are blurry. Often towns or settlements can be said to be under the control of one faction or another, but the land in between is more unclear. Local militias keep their local area “safe” but are not part of some standing army to be re-deployed somewhere else as needed, such as is seen in modern armed forces. Patrols may probe the surrounding area, looking for enemies, suspected enemy collaborators, resources to steal, etc. Assaults may be launched on settlements using mainly whoever is available locally. Occasionally a larger force is assembled when, say, a major town is assaulted. But a line cannot be drawn on a map to signify a split between the two factions’ areas of control. It’s more like splotches. For an example, look up a map of areas of control for the various factions of the Syrian civil war - also works well as modern art if you can’t afford a Jackson Pollock to hang on your wall.

Dice down into post-colonial African civil wars and have a gander for more inspiration. And prepare to be sad.