Caches

Is there any in game or lore reasons why factions get caches after trying to pummel each other to oblivion?

It's war and the cost of war is high. You got revives and mercs, xan stacks, it's costly so caches are the reward . 

 You can make good money but it costs to go to war

The expended ammo gets planted in the ground and then watered with blood. After the war it blooms into a nice little cache tree.

After realising Torn City was lost to the criminal organisations that roamed free the US gave it independence.

However this then posed a new problem, due to the expansion of the criminal networks there was a deep seated fear from the surrounding US population that the city boarders wouldn't be enough and the people of torn would expand outwards.

As a result of this the US government made a gamble that has, so far, paid off. Factions that war eachother would be entered into a league table with amour and weapons being provided as the prize, the higher you climbed in this, the larger your reward would be. 

To ensure maximum effectiveness the US government allowed both participating sides to receive a reward as long as they actively took part. The idea was this would stop larger organisations just trampling over everyone.. unfortunately this is where the plan backfired a little as many factions termed their wars, minimising damage to eachother but still being entitled to the prize.

Overall the goal was achieved, but time will tell if it's enough to stop to expansion of the map