There's a huge I/O bottleneck as well as you're reading huge textures (I've seen textures as big as 1 TB) and writing constantly to disk the result of the renderer.
Other than that, most of the tooling that modern studios use is off the shelf, for example, Autodesk Maya for Modelling or Sidefx Houdini for Simulations. If you had a custom architecture then you would have to ensure that every piece of software you use is optimized / works with that.
There are studios using GPUs for some workflows but most of it is CPUs.
This is probably true today, but leaves the wrong impression IMHO. The clear trend is moving toward GPUs, and surprisingly quickly. Maya & Houdini have release GPU simulators and renderers. RenderMan is releasing a GPU renderer this year. Most other third party renderers have already gone or are moving to the GPU for path tracing - Arnold, Vray, Redshift, Clarisse, etc., etc.