From previous experience of modding various games, it is sometimes the case that certain features exposed for modding are closed back into the box because of plans for future content additions to create new income streams that do not have free alternatives provided by mods.
This is speculation and it is not a judgement, simply a reality of the gaming industry. Of course, your own speculation is just as likely correct.
edit - It should also be noted that sometimes this happens for the very legitimate reason of maintaining high game performance, especially use of the CPU. Still, there's plenty exposed to modding that could very easily bring the game to its knees when poorly implemented.