You can't put the AI in the soft-code any more than you can put the graphics engine. What modders might be able to change is some the various thresholds, yes/no options and modifiers/multipliers the AI uses, but the main structure of the AI code needs to be hardcoded. And programming good AI, finding the perfect balance between performance and quality, is very hard (it's nothing modders can do, it's not like changing 2d or scripting, one would need a degree in informatics at least) and money-intensive. And of course it's a fact that AI is not as cost-effective for game developers as shiny graphics are.