As you know we do have weight/modifiers/"triggers" AI on e.g. EU4 but it's usually not terribly amazing (an understatement), though indeed modders end up tweaking it way better than the scripted developer AI in the long run.
The main issue here is always the interface between C++ and whatever scripting/data language is used. There's no solution that works best for multiple objectives in software engineering, and it turns out that very moddable AI in an otherwise closed game (where gameplay code resides in C++) majorly adversely affects AI programmer agility and productivity, as well as quite often performance. Sometimes certain details can be put in script because it's convenient to do so, and at other times (more depressingly) because other moddable parts of the game makes having any sort of specific assumptions on the gameplay impossible from AI code, usually leading to poor AI.
Anyway sometimes you're in luck as a modder and it was easier for the developer to put AI in script than not.
