The programmers at CA couldn't give the AI good enough tactical or strategic awareness in order to really challenge the player. Therefore, on harder difficulties, the AI gets a ton of money and builds, builds, builds, but doesn't do much with it.

For example, as the SPQR, I usually end up fighting a long, protracted border war full of minor skirmishes with Epirus, resulting in me devastating their military, while training mine up to multiple silver chevrons. They keep pumping troops out. If I were stupid, I could march right on Tarantum, and probably win without too much difficulty, but that wouldn't make my army all that loyal, or give my commanders much field experience. When I finally take on Carthage, I usually have three stacks full of experienced legions to conquer Sicily with.