I suspect geometric kernels and 2D/3D renderers don't fall into the "easy to parallelize" category. Of course there are functions that use multiple threads, but it's not obvious how you could build the core system to do so. However the code in CAD software is often pretty old, it wasn't that long ago that many of these still used intermediate mode OpenGL and I wouldn't be surprised if some still do.
In the same vein something like ECAD tools don't use GPU-accelerated 2D rendering but instead use GDI and friends (which used to be HW-accelerated, but isn't since WDDM/Vista).
A lot of "easy" opportunities to improve UX and productivity.