Fighting costs are intended to control expansion rates. Because you get more city expansions for clearing more provinces, the developers need to control how quickly you obtain those. If you hover over the Advanced scouting chest that opens the next chapter in your tech tree, you'll see the number of provinces you have finished vs the number required to advance to that province. If you are over by very much (more than 10 or so) you are scouting faster than the game designers want you to, and costs will continue to be high. If you are in Era 3, then you only need 50 provinces to advance to Era 4. More than about 60, and the developers have no interest in making it easier.
You might want to turn your attention to the tournaments if you enjoy the fighting.