Because it will be a lot of work. I assume they will need to change the rig so it can be dynamic, and on top of that, they will need to re-check all the clipping issues that will occur with objects, clothes, many issues. Keep in mind, in TS4 all the objects are designed with specific seamless dimensions so they can fit with all the animations from the models. And other issues with the environment and whatnot.
It is not worthy to use any mod unless you dont care about animations going awol and other issues (and honestly for me clean animatios are essential on this game).