Pixar renders their movies with their commercially available software, Renderman. In the past they have partnered with Intel [1] and Nvidia [2] on optimizations
I'd imagine another reason is that Pixar uses off-the-shelf Digital Content Creation apps (DCCs) like Houdini and Maya in addition to their proprietary software, so while they could optimize some portions of their pipeline, it's probably better to develop for more general computing tasks. They also mention the ability to "ramp up" and "ramp down" as compute use changes over the course of a show
[1] https://devmesh.intel.com/projects/supercharging-pixar-s-ren...
[2] https://nvidianews.nvidia.com/news/pixar-animation-studios-l...