On the other hand modding the AI in Rome went only as far as changing the formations & formation priorities in the relevant text file (which helped somewhat but not nearly enough to cure the AI of it's brainless habits) and this was due to the diligent efforts of one person (Darthmod) and his personal crusade in making the AI put up a better fight. So comparatively speaking changing graphics/3D models = easy, changing gameplay/AI = hard if not impossible.
Very few modders are looking at the AI, because so many are still busy with the graphics. It is easy if you have the skills and the tool, but that doesn't mean that it is a piece of cake to fix hundreds of flaws made by CA. That takes alot of time, just look at how long EB or any other great mods are taking to finish. Darth Vader did make any graphical changes, or he took them from other mods/modders, that is why he was able to focus on AI, it was the only thing he had attention for.